ci: Don't cancel gstreamer MR pipelines when pushing to cerbero main
We mark all cerbero jobs as interruptible, which is good because that means pushes to cerbero MRs that obsolete commits in those MRs will not keep running stable jobs / pipelines.
However, this has an unfortunate side-effect that pushes to cerbero main cancel the cerbero pipelines triggered by merge requests in the gstreamer monorepo and causes CI on MRs to fail.
Mark cerbero trigger jobs as non-interruptible.
The whole interruptible thing was useless for triggered pipelines anyway. When you cancel a pipeline on a gstreamer MR, it does not cancel the trigger cerbero pipeline. For that we need https://gitlab.com/gitlab-org/gitlab/-/issues/15603 and add a script that manually cancels the cerbero trigger pipeline.