ci: Don’t run pipeline after merging a MR

MRs are already tested in CI before merge, so it’s redundant and a waste
of resources to test them again after merge.

In the rare case where something breaks post-merge (perhaps because
several MRs have been tested individually and merged, but interact with
each other badly), that’ll be caught in the weekly scheduled CI run.

YAML inspiration from https://stackoverflow.com/questions/63893431/gitlab-run-a-pipeline-job-when-a-merge-request-is-merged

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -19,11 +19,18 @@ variables:
   G_MESSAGES_DEBUG: all
   MESON_COMMON_OPTIONS: "--buildtype debug --wrap-mode=nodownload --fatal-meson-warnings"
 
+# Default CI job setup; contrast with `.only-origin`.
+#
+# Don’t execute the pipeline when a merge request is merged into `main`, as it
+# will have already been tested
 .only-default:
   only:
     - branches
   except:
-    - tags
+    refs:
+      - tags
+    variables:
+      - $CI_PIPELINE_SOURCE == "push" && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
 
 # Some jobs run on CI runners which don’t have good isolation between CI jobs,
 # and don’t have much available resource. Limit those jobs to only ones on the