Fix behaviour of pipelines when [skip ci] is used

Apparently there has been a change in the gitlab policy where, if we enforce that pipelines must succeed there must always be a CI job that runs successfully, so we might have to implement something like https://docs.gitlab.com/ee/user/project/merge_requests/merge_when_pipeline_succeeds.html (see Limitations). https://gitlab.cern.ch/help/user/project/merge_requests/merge_when_pipeline_succeeds.md#only-allow-merge-requests-to-be-merged-if-the-pipeline-succeeds

Associated gitlab issues describing the policy change: https://gitlab.com/gitlab-org/gitlab/issues/14791 https://gitlab.com/gitlab-org/gitlab-foss/issues/66271

Affects !180 (merged) see also !184 (closed)