build-system/ruby: Add #:gem-flags parameter.

* guix/build-system/ruby.scm (build): add 'gem-flags' key
* guix/build/ruby-build-system.scm (build): use 'gem-flags' key
* doc/guix.texi (Build Systems): Mention #:gem-flags.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>