[ci] streamline logic between scripts

.gitlab-ci/default.yml

@@ -23,9 +23,10 @@ select tests:
   before_script:
     - |
       DUMUX_CLONE_BRANCH=master
-      if [[ -n "${DUMUX_MR_SOURCE_BRANCH}" ]]; then
-        DUMUX_CLONE_BRANCH=${DUMUX_MR_SOURCE_BRANCH}
+      if [[ "$DUMUX_PIPELINE_SOURCE" == "merge_request_event" ]]; then
+        echo "Starting pipeline triggered by upstream merge request event"
         echo "Received upstream merge request branch ${DUMUX_MR_SOURCE_BRANCH}"
+        DUMUX_CLONE_BRANCH=${DUMUX_MR_SOURCE_BRANCH}
       fi
     - echo "Checking out branch ${DUMUX_CLONE_BRANCH} in dumux"
     - git clone -b ${DUMUX_CLONE_BRANCH} --depth 1 https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git