Add CI variable support to fork_group module; set for ruby/gems

Asherah Connorrequested to merge
add-ci-variables-to-fork-group into main

Hi! The FF_DISABLE_UMASK_FOR_DOCKER_EXECUTOR CI variable/GitLab runner FF is set for the gitlab-org/ruby/gems group, and our CI jobs depend on that (otherwise files end up getting created with 0666 (!)).

This variable isn't set in the community forks, however, so the fork pipelines fail when they check file permissions. Many thanks to @_stingrayza's investigation in #co-create-and-community-engineering for surfacing this!

This MR is Duo-created and intends to set the same variable on the gitlab-community/ruby/gems group. There wasn't plumbing to set CI variables yet, so we've added some.

Edited by Asherah Connor

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