ci: blackduck: update blackduck scan job trigger

Update pipeline rules to run only on:
- Protected branches with tags
- Scheduled pipelines
- Merge request events (manual trigger)

References: PCF-1794
Signed-off-by: Matthias Franck <matthias.franck@softathome.com>

.gitlab/blackduck.yml

@@ -4,7 +4,10 @@
   timeout: 4 hours
   needs: []
   rules:
-    - when: manual
+    - if: $CI_COMMIT_REF_PROTECTED && $CI_COMMIT_TAG
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+    - if: "$CI_PIPELINE_SOURCE == 'merge_request_event'"
+      when: manual
   before_script:
     - touch .build_failed
     - export CI_BUILD_PROFILE="$(echo $CI_JOB_NAME | sed 's/blackduck scan \(.*\)/\1/')"