Unify pipelines for dumux-course

Currently the pipeline stage apply_patches is executed in another pipeline compared to the ones initiated by the stage trigger pipelines. As a result, when running pipelines in merge requests there are always two pipelines, one for apply_patches and one for the testing. As the stage apply_patches is not part of the testing pipeline, the merge request can be merged in some cases if the the stage apply_patches fails.
Possible approaches: Avoid running the stage apply_patches on all triggers, but maybe reduce it to a rule like - if: $CI_PIPELINE_SOURCE == "merge_request_event". Also think about unifying the different pipelines.