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/README.md

@@ -259,7 +259,7 @@ details.
   Override the number of jobs to run simultaneously, you might want to do so on
   a memory-limited machine. This may be passed to:
 
-  - `guix` build commands as in `guix environment --cores="$JOBS"`
+  - `guix` build commands as in `guix shell --cores="$JOBS"`
   - `make` as in `make --jobs="$JOBS"`
   - `xargs` as in `xargs -P"$JOBS"`
 
@@ -301,7 +301,7 @@ details.
 
 * _**ADDITIONAL_GUIX_ENVIRONMENT_FLAGS**_
 
-  Additional flags to be passed to the invocation of `guix environment` inside
+  Additional flags to be passed to the invocation of `guix shell` inside
   `guix time-machine`.
 
 # Choosing your security model