Pass CACHE_UPDATE variable to omnibus-gitlab-mirror if present

Note: We have to set CACHE_UPDATE variable to true in the [2-hourly] Repo caching, Review Apps cleanup, Caches update schedule before/after this MR is merged.

Make the 2-hourly pipeline (from schedule [2-hourly] Repo caching, Review Apps cleanup, Caches update) cause omnibus-gitlab-mirror to update its package build cache so that MRs from this project can see faster builds.

Related to omnibus-gitlab!5528 (merged) where we are making pipelines from this project's MRs use a different cache key.