citemplate: tweak merge-request rules

citemplate: tweak merge-request rules

With the MergeRequest rules template, its was impossible
to manually create or trigger a pipeline for a branch/commit
that was not part of a merge request.

This tweaks the workflow:rules such that there will always be
a pipeline for each commit (which will be set to a manual job trigger).

There is the downside that we will now be triggering 2 pipelines in
case of merge requests, but they will require manual action to be
started.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/369>