Rename ‘environment.scm’ to ‘guix.scm’ and recommend ‘guix shell’.

This follows current upstream recommendations.

* environment.scm: Rename to…
* guix.scm: … this.  Mention ‘guix shell’ rather than ‘guix environment’.
* Makefile.am (EXTRA_DIST): Add it.
* README.org: Update instructions to use ‘guix shell’ rather than ‘guix
environment’.
* .gitlab-ci.yml (build): Update.