Add Ruby gem and Gitaly binary cache CI jobs for FIPS

What does this MR do and why?

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 (gitlab-build-images!775 (merged)).

Relates to #441519 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.