[ci] make lecture trigger manual

.gitlab-ci.yml

@@ -131,6 +131,7 @@ trigger lecture:
   - if: $CI_PIPELINE_SOURCE == "schedule"
   - if: $CI_PIPELINE_SOURCE == "pipeline"
   - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    when: manual
   stage: trigger dumux pipelines
   trigger:
     project: dumux-repositories/dumux-lecture