Fix check for branch protection state
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.