Add validation tag to commits (alternative)

Similar to !8 (merged), but instead of skipping jobs, we just pass then using a before script.

This alternative allows MRs to be approve, because the pipeline "succeeds", even though the jobs themselves don't run.

However the job still download dependencies for each job, which take some time, but should be trivial against the time of running the actual job.

Part of #8 (closed)