guix: switch from guix environment to guix shell

See https://guix.gnu.org/manual/devel/en/html_node/Invoking-guix-environment.html.

> Deprecation warning: The guix environment command is deprecated
in favor of guix shell, which performs similar functions but is more
convenient to use. See Invoking guix shell.

> Being deprecated, guix environment is slated for eventual removal,
but the Guix project is committed to keeping it until May 1st, 2023.
Please get in touch with us at guix-devel@gnu.org if you would like
to discuss it.

See also https://guix.gnu.org/blog/2021/from-guix-environment-to-guix-shell/
for a blog post and additional details.

Guix shell was added to guix in this commit,
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=80edb7df6586464aa40e84e103f0045452de95db
which isn't part of the 1.3.0 release binaries out of the box, but
invoking a guix pull, and updating will make it available for all
builders.

contrib/guix/guix-codesign

@@ -286,7 +286,7 @@ EOF
         # Run the build script 'contrib/guix/libexec/build.sh' in the build
         # container specified by 'contrib/guix/manifest.scm'.
         #
-        # Explanation of `guix environment` flags:
+        # Explanation of `guix shell` flags:
         #
         #   --container        run command within an isolated container
         #
@@ -343,7 +343,7 @@ EOF
         #    more information.
         #
         # shellcheck disable=SC2086,SC2031
-        time-machine environment --manifest="${PWD}/contrib/guix/manifest.scm" \
+        time-machine shell --manifest="${PWD}/contrib/guix/manifest.scm" \
                                  --container \
                                  --pure \
                                  --no-cwd \