ci: trigger required checks on merge_group events (#879)

Without merge_group on the workflow, PRs entering the merge queue would
fail because no required check ever reports a status against the
merge_group ref.

.github/workflows/ci.yml

@@ -5,6 +5,7 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  merge_group:
   workflow_dispatch:
 
 jobs: