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