$ guix shell --dry-run cowsay
hint: Consider passing the `--check' option once to make sure your shell does not
clobber environment variables.

The following derivations would be built:
  /gnu/store/px96xflxvak6q254m3vygwk3qjj92n8j-glibc-utf8-locales-2.33.drv
  /gnu/store/6y82rf0nqzaa20d9973yxy87qnmh253g-guile-3.0.7.drv
  /gnu/store/0z2136503g3kwvpg57kadsjnxnpay1hd-patch-boot0-2.7.6.drv
  /gnu/store/19yddflnvhdaii0nr60c3gn74x3nncxr-glibc-mesboot-2.16.0.drv
  /gnu/store/3fq815zsrhjfyfpcpp3bcn1bqs3xk2w4-xz-mesboot-5.0.0.drv
  /gnu/store/zkp45dm2zwkhzfnrbqdhx7zahhzjywkr-gcc-mesboot1-4.6.4.drv
  /gnu/store/css2dzk2y9bj60lzhgzk1khn2rqgz6vz-binutils-mesboot-2.20.1a.drv
  /gnu/store/zd95sgxaj5lw2ldbpv44lkbnp8b3wh7d-glibc-headers-mesboot-2.16.0.drv
  /gnu/store/3z3pfhimvvd41qkc52vl18vnimq26sbh-make-boot0-4.3.drv
  /gnu/store/a0l5jw0ppc3lhy64pv85bsryld6ig1c6-gcc-mesboot-wrapper-4.9.4.drv
  /gnu/store/q41kglwbnznkscnppdynw364a2dshwk5-gcc-mesboot-4.9.4.drv
  /gnu/store/4bj590nw0p2jzrq51dd1zbs871f74wn0-gcc-mesboot1-wrapper-4.6.4.drv
  /gnu/store/f2xh0fdfxph54lm1z0nxv2611f431k5b-coreutils-mesboot-8.32.drv
  /gnu/store/304ndyrxkkbpgnvdp5jq8zg25vpplh99-libgc-8.0.4.drv
  /gnu/store/49z5gaws058axmay4wddlf2006qz9czf-tar-boot0-1.34.drv
  /gnu/store/6rg9q4hvygbivcdl37hi5dad9506153b-findutils-boot0-4.8.0.drv
  /gnu/store/zc7ls1g5pwndz52h43q708lz9x1yqind-diffutils-boot0-3.8.drv
  /gnu/store/8n17jxr2199fm963q3z2z1k3xnin3isj-bzip2-boot0-1.0.8.drv
  /gnu/store/cspldvr61pfhbv9lx17qz2bv1f9zd45n-file-boot0-5.39.drv
  /gnu/store/cv4z4bjdd33hq78djsqiir6sa761rh2i-ld-wrapper-boot3-0.drv
  /gnu/store/74d8zi0c41bqrbrn0d2acchv712liycd-binutils-2.37.drv
  /gnu/store/069ns63msjm8qfryxy5w2hs92ivqfplz-gcc-cross-boot0-wrapped-10.3.0.drv
  /gnu/store/czk1ywrqs34miwf32n998didr9djx5i1-gcc-cross-boot0-10.3.0.drv
  /gnu/store/jwpx7r1vd7qcg08s68wb76jziam83jfj-binutils-cross-boot0-2.37.drv
  /gnu/store/mm563p81j5cqby9igbjk8830r3ni05gk-coreutils-boot0-8.32.drv
  /gnu/store/qp3bbq3x0bl9hdrp64rmwm3kb5npz7qd-gawk-boot0-5.1.0.drv
  /gnu/store/zwdjn6hb82d5vajylznq0dz3933njjcq-sed-boot0-4.8.drv
  /gnu/store/liryb4j5pgrkb29jmkhsa6bxk3qrmxwz-libstdc++-boot0-4.9.4.drv
  /gnu/store/divi9yclhgmgyi4qgry8vskwypvqydm2-glibc-2.33.drv
  /gnu/store/09fsr0fikzbggdxn1fplkq802n4a9i20-m4-boot0-1.4.18.drv
  /gnu/store/2g4kbasz94wqdrqdkk8mzmcs8kl6q2zb-bash-static-5.1.8.drv
  /gnu/store/8dpvs363q0n4xfg05ixscnh4p6f6gzys-glibc-intermediate-2.33.drv
  /gnu/store/4782psmwzljrjy7232ybbq7i6984gh9f-python-minimal-3.5.9.drv
  /gnu/store/p73awjmhjvn1x1ah150xwxpcah64lbz4-expat-2.4.1.drv
  /gnu/store/kj0f6b86s44khi0w10d9hrj1yx68144l-perl-boot0-5.34.0.drv
  /gnu/store/l4n8nhdlqz1gx2pszvwvyzqy1vvbmx5i-ld-wrapper-boot0-0.drv
  /gnu/store/n3i9ndycicyf4sn3px6c1vvkcscw1lvm-texinfo-6.7.drv
  /gnu/store/q1jrcg84829mcs6pwlyp94zgykfkjbbi-linux-libre-headers-5.10.35.drv
  /gnu/store/mpbmp5yf2jsazr52zaiddzaz1zpli685-flex-2.6.4.drv
  /gnu/store/ya78hn7zfn06nmd0b5wcbk4hrfpbdy8b-bison-boot0-3.7.6.drv
  /gnu/store/yz4gmx41bihgzj1yrsp0svhhjfh1qvwl-gcc-cross-boot0-wrapped-10.3.0.drv
  /gnu/store/8dpvs363q0n4xfg05ixscnh4p6f6gzys-glibc-intermediate-2.33.drv
  /gnu/store/r6filibyfp5va4gx36szn5ynqxffvrnf-gettext-boot0-0.19.8.1.drv
  /gnu/store/divi9yclhgmgyi4qgry8vskwypvqydm2-glibc-2.33.drv
  /gnu/store/ixf06cfzi96walzqcr0pyf868xyg9vwl-pkg-config-0.29.2.drv
  /gnu/store/jlrywvyz6p6ny8rzyijwdyiwh6jb2gg8-bash-minimal-5.1.8.drv
  /gnu/store/kivh37q1q1mn2pg258w0g5zby97bvaxp-gcc-10.3.0.drv
  /gnu/store/k5bg1h70g59zzmkfnnnkc4n37acbkmah-zlib-1.2.11.drv
  /gnu/store/vf8q2gzsankp1i9f144maz20a8djmyg0-libstdc++-10.3.0.drv
  /gnu/store/dxqfh86kdmjvmyzgpscwn7k1a1ihvxb3-libffi-3.3.drv
  /gnu/store/r655i42ig0sz5sviydfl7b9ll433n1qv-libunistring-0.9.10.drv
  /gnu/store/g933yags7m10vv76518vf19fgr05c7fs-linux-libre-headers-5.10.35.drv
  /gnu/store/4avbbjshvijicbpx260kqc9c8lj9qzwy-gzip-1.10.drv
  /gnu/store/5cw6i6a6varhrvc2xs6rwqk62rc73qdr-coreutils-8.32.drv
  /gnu/store/93ngmg4q9w19n12ja4mg3aaygf5w1p76-glibc-utf8-locales-2.33.drv
  /gnu/store/60i6n2g89fn973dkhvnm2cpwgvbszy8g-gzip-1.10.drv
  /gnu/store/qcnbyff3nj4rl5j78f0qsh43wz67df96-module-import-compiled.drv
  /gnu/store/k3gyb35i614zpsbgl74adx764fcw81qx-module-import-compiled.drv
  /gnu/store/y5n159fagf3mivxcmc53limd28jcrhpr-grep-3.6.drv
  /gnu/store/4qdzycn4w5x1g09afjk5w782f8smb97i-m4-1.4.18.drv
  /gnu/store/fdk3f4nyz1xzyv7bwzx0cy571lzscl90-m4-1.4.18.tar.xz.drv
  /gnu/store/g469q61iiq2s2pfaij3ndrx4zsricm17-patch-2.7.6.drv
  /gnu/store/p6rrljwb23acfrrqla2r915pqspm88kq-ed-1.17.drv
  /gnu/store/z2rc0hlkl0ssqh6xliry2v4varrgixrd-lzip-1.22.drv
  /gnu/store/qxp919qw4y12vi9cj1pi9zphfwnriwc9-tar-1.34.drv
  /gnu/store/s0c9mcz7z4c5rkim763bfhv6zdwl277z-xz-5.2.5.drv
  /gnu/store/fiiswyx9k0qfs54ypnxy2n4d88m8cva9-file-5.39.drv
  /gnu/store/ii0l2p2dapackc0w1q88qgfhp348l01r-sed-4.8.drv
  /gnu/store/m82q4idyq9hpmg1ds55qcj4zm0b2hizc-gawk-5.1.0.drv
  /gnu/store/l7qg2n5ycq4vwwhxj3yqj41qag5b6dfk-libsigsegv-2.13.drv
  /gnu/store/q9gahridzyx2ncx84xrpnqripfbpvxrk-ld-wrapper-0.drv
  /gnu/store/scd62mivg0lqdg1gji200wphfdxcjvzl-make-4.3.drv
  /gnu/store/d51cll7fp4wf0ylg86216305kd9kkw9n-pkg-config-0.29.2.drv
  /gnu/store/vkwzs5gs1f239zc03mxkfxw5c92lyaa4-findutils-4.8.0.drv
  /gnu/store/vs4hcxjaswj7v0gir6sv0zfdr7cwirj8-bzip2-1.0.8.drv
  /gnu/store/xfvqic30id7d5lrkz71iaprvzv2wr8n4-diffutils-3.8.drv
  /gnu/store/bz31gx79nyfzv0gx8zikcwcr2vyp2rp6-perl-5.34.0.drv
  /gnu/store/5g1cbmwx01awfsg18g33dxgdkq5fkdag-coreutils-minimal-8.32.drv
  /gnu/store/6a3flfm4wiw5zyqddlixycrgpxzi8bjw-bison-3.7.6.drv
  /gnu/store/7fb4wf6s99fyis9g0l6sdgl50sdjz0cz-flex-2.6.4.drv
  /gnu/store/67l3anhc8g6z90vzn76gdqz3ivk0yc36-bison-3.7.6.drv
  /gnu/store/cw53986ag5jd647rmqlk0fvpkwmky133-perl-5.34.0.drv
  /gnu/store/6bc2kxwl0dy56rjrabxs09ihplg3xbyl-perl-5.34.0.tar.xz.drv
  /gnu/store/p4c44jcn5f18x4mznzamkfixb4aaz7f6-coreutils-minimal-8.32.drv
  /gnu/store/j6mwslxb7prqapyhznxcknys9nm11fdj-coreutils-8.32.tar.xz.drv
  /gnu/store/9zgczjm2zm8c7pliv3jkyzib1l8calci-help2man-1.48.5.drv
  /gnu/store/11hfh4d4i63wb0i67d8zp7d60kmd44y6-perl-gettext-1.07.drv
  /gnu/store/r771lw5pf8ac287sq1bnd738dv7jjfas-module-import-compiled.drv
  /gnu/store/f3fkm5cz6xcqplfydwrigjqjs0xr7jpx-gettext-minimal-0.21.drv
  /gnu/store/9g0j8nwp1v5ggr8wlybarkaz200iicgs-zlib-1.2.11.drv
  /gnu/store/9l2zrsa8abx4wd2cqsjd4yzfy9gpv93l-libunistring-0.9.10.drv
  /gnu/store/f5vadd5q112bbvnypq7fi4iwjpxdlgv6-ncurses-6.2.20210619.drv
  /gnu/store/yw4igqx7h8iybd6z78fyk4y6ldlshzhr-pkg-config-0.29.2.drv
  /gnu/store/vv6acn4a9l6b4zfcv03qsm8h25hb0qi7-libxml2-2.9.12.drv
  /gnu/store/ashb7vn2823091iyhlqpd5k96ph5i08r-libxml2-2.9.12.tar.xz.drv
  /gnu/store/mrq9hnkhcfdb3vyzf79k4v9i5dnjj6f1-xz-5.2.5.drv
  /gnu/store/mf9r4rrqc8k3fal7qgjqpk216ylf14wl-gzip-1.10.drv
  /gnu/store/zzzkix509asnfgpgh9f6fm8y4xa5cf9z-glibc-2.33.drv
  /gnu/store/03fx51ycqhdjdzr94z8bk7zlq1zp3gbw-python-minimal-3.9.9.drv
  /gnu/store/4kxk7p2l334sd3xwhymsbighin94c6fc-openssl-1.1.1l.drv
  /gnu/store/lw2zwhxi6a8fkv72hddk1v8yyvlrn6ln-openssl-1.1.1l.tar.xz.drv
  /gnu/store/a4hhdwkj07zflr2ll4iksx0y27481d3m-expat-2.4.1.drv
  /gnu/store/dhyb73ff8v8i0k8z9slwivdz2slxzrrq-libffi-3.3.drv
  /gnu/store/szfcw4wsnkbb5iadlaky8ndm5j6ycnz5-libffi-3.3.tar.xz.drv
  /gnu/store/n7r0hiifmy6wbrl6hwhwwnp9jzgz5swz-Python-3.9.9.tar.xz.drv
  /gnu/store/p6b8q13n4507a65wmqk5f0i53gg3hdf0-tzdata-2022a.drv
  /gnu/store/p85sv30vfkjpjwzwx8hipkdz0l1cfrpm-unzip-6.0.drv
  /gnu/store/42z7m8wbz4nrhgvvny692l0nml2zjc2w-bzip2-1.0.8.drv
  /gnu/store/wn38piyrbdly5126kw0vxl0p8c1y4zmv-unzip60.tar.xz.drv
  /gnu/store/y5kffkiyjaraih9rvwr241r59siz6nah-zip-3.0.drv
  /gnu/store/93qc3kj27q1mlgv4mr02fzbn72dda2f2-bash-static-5.1.8.drv
  /gnu/store/f719gaw5ngvzyrdbdf6a4486bhvrrlhk-bash-5.1.tar.xz.drv
  /gnu/store/kf6hsgbzqg7wzgkkiyf38kmfw401bjy7-glibc-2.33.tar.xz.drv
  /gnu/store/xilj2n7w1n6dwyrcp64r7zirbmkmyqb9-texinfo-6.7.drv

634.8 MB would be downloaded
$ /etc/guix/acl
(acl
 (entry
  (public-key
   (ecc
    (curve Ed25519)
    (q #8D156F295D24B0D9A86FA5741A840FF2D24F60F7B6C4134814AD55625971B394#)
    )
   )
  (tag
   (guix import)
   )
  )
 (entry
  (public-key
   (ecc
    (curve Ed25519)
    (q #7D602902D3A2DBB83F8A0FB98602A754C5493B0B778C8D1DD4E0F41DE14DE34F#)
    )
   )
  (tag
   (guix import)
   )
  )
 )
$ cat .config/guix/channels.scm
(use-modules (guix ci))

(list (channel-with-substitutes-available
       %default-guix-channel
       "https://ci.guix.gnu.org"))