build: Use guix shell.

The recently introduced 'guix shell' command automatically manages a
cached profile for us; prefer it to 'guix environment', and do away
with a channels file, given fixes in Guix have now been incorporated
in the main branch.

* Makefile (portable-release-tarball)<guix environment>: Replace by
'guix shell'.
* build.py (run_dependencies)[guix]: Likewise, and do not use a
channels.scm file.
(run_install): Likewise, and provide a hint when TARBALLS is not set.
* guix/channels.scm: Delete file.
* guix/manifest.scm: Adjust comment.

Change-Id: I50ca99e51f0ebca136c9bbc5a0ca3063075432b4