Add Ruby gem and Gitaly binary cache CI jobs for FIPS

This will allow the FIPS jobs to run without having to reinstall
Ruby gems and recompile Gitaly and Workhorse.

Replace `DEBIAN_VERSION` and hard-coded `debian` values in favor
of `BUILD_OS` and `OS_VERSION` variables.

Simplify the `setup-test-env-fips` CI step now that `libpq-devel` is
now in the UBI image
(https://gitlab.com/gitlab-org/gitlab-build-images/-/merge_requests/775).

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/441519