Keep a copy of gems needed by our releases as CI artifacts

This is meant as insurance in case a gem GitLab needs is ever removed
from rubygems.org. After we merge this, all future GitLab tags will
have a CI artifact archive containing all the gems (for all platforms)
specified in Gemfile.lock for that revision.

Only runs on tags (so we don't constantly create 50MB gem cache bundles).

See merge request !6617