Since the changes of 939ade2b we observe
that pipelines are executed on non-protected branches, which is not what
we want.

GitLab's documentation was a bit unclear on the according CI variables
CI_COMMIT_REF_PROTECTED and CI_MERGE_REQUEST_TARGET_BRANCH_PROTECTED, as
stated in this stack overflow answer:
https://stackoverflow.com/a/59023344/3018229

We need to explicitly check for the variable values to be "true" instead
of just checking whether they are set.