* guix/scripts/system.scm (perform-action): Add #:derivations-only?
parameter and honor it.
(show-help, %options): Add '--derivation'.
(guix-system): Pass #:derivations-only? to 'perform-action'.
* tests/guix-system.sh: Test it.
* doc/guix.texi (Invoking guix system): Document it.