ci: fix pipeline run for merge requests

Merged Tim-Philipp Müller requested to merge tpm/opus:ci-fix-pipeline-for-merge-requests into master

This way CI pipeline runs for branches and tags and makes it show up in merge requests where a branch is used as source branch.

Makes all jobs show up in merge request CI indicator (not just the whitespace job) and only runs a single CI pipeline, without the additional detached pipeline.

https://docs.gitlab.com/ce/ci/yaml/#workflowrules-templates

Edited by Tim-Philipp Müller

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