Usage of ONLY_JOBS pipeline may prevent marge from merging the patch

!13350 (closed) added support for using ONLY_JOBS to run only specific jobs in a manually triggered pipeline. However, this may prevent marge from merging the patch: if the normal MR pipeline passes and the manually triggered pipeline fails, marge would consider the MR to have failed CI. However the manual pipeline is only meant to be a debugging tool and shouldn't be taken into account by marge, and it can fail for very good reason (e.g. doc-tarball fails if windows job is not run which is often not needed in such pipelines).

The culprit is https://gitlab.haskell.org/ghc/marge-bot/-/blob/master/marge/job.py?ref_type=heads#L128. We should filter pipeline source to merge request and not just pick yet another pipeline that fits the git revision of specified merge request.