ci: avoid spawning 2 pipelines for each commit
When an MR is open, you can have both a normal and a "detached" pipeline.
They have different properties, but a detached pipeline is enough for us and identical to the normal one, so we can avoid the extra load on the CI.
https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines
Edited by Sebastian Dröge