ci: Do not create pipelines for branches with open MRs

ci: Do not create pipelines for branches with open MRs

Pipelines for non-protected branches are set to 'manual', and
thus cheap. However they may still get picked by `@marge-bot`,
meaning that the bot waits for the completion of a pipeline that
never starts.

Avoid that by not creating pipelines for branches with open
merge requests.

Credit to Jordan, who came up with this for gst.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3153>