Fixes a bug whereby ‘package->development-manifest’ would run with the
wrong system in mind, leading to errors like this:
$ guix shell -s i586-gnu -D shepherd --no-grafts
guix shell: error: package linux-libre-headers@5.15.49 does not support i586-gnu
* guix/scripts/environment.scm (options/resolve-packages): Define
‘system’ and pass it to ‘package->development-manifest’.’
* tests/guix-shell.sh: Test it.
Change-Id: I95c471c1918913ab80dec7d3ca64fe38583cce78