Gitlab CI: do not trigger plugin pipelines if run on schedule

This restores the previous behavior. The plugin project has not tests
that benefit from running even if there are no changes to the Maven
artifacts of this project.

Author: greenrobot-team

.gitlab-ci.yml

@@ -192,6 +192,9 @@ trigger-plugin:
     # Do not trigger publishing of plugin
     - if: $CI_COMMIT_BRANCH == "publish"
       when: never
+    # Not for scheduled pipelines where Maven snapshots of this project do not change
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      when: never
     # Otherwise, only if no previous stages failed. Also set allow_failure in case branch does not exist downstream.
     - when: on_success
   inherit: