CI: Optimize pipeline for merge train setup

Doing the ohos build test with oniro-openharmony-toolchain and
oniro-openharmony-bundle images is rather expensive, so we will not run them
automatically until the final merge train pipeline.

This way we can still ensure that we don't merge anything that breaks this,
while allowing faster pipeline execution during branch and merge request
iterations.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

.gitlab-ci.yml

@@ -361,8 +361,9 @@ build_ch:
 # URL and filename for tarball containing entire OpenHarmony OS codebase
 .openharmony_tarball:
   rules:
-    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-    - if: $CI_COMMIT_BRANCH
+    - if: $CI_MERGE_REQUEST_EVENT_TYPE == "merge_train"
+      # Only run these rather expensive jobs in the merge train pipelines
+    - if: $CI_COMMIT_BRANCH || $CI_PIPELINE_SOURCE == "merge_request_event"
       when: manual
       # We don't really want these jobs to fail, but we don't want to block
       # pipeline and dependent jobs either