Enable testing merge queues @ GitHub Actions CI/CD

This allows org-hosted projects to start enabling merge queues in the
repository settings. With that, GitHub would trigger a separate event
against a merge commit derived from merging several pull requests with
the target branch.

.github/workflows/main.yml

@@ -1,6 +1,6 @@
 name: tests
 
-on: [push, pull_request]
+on: [merge_group, push, pull_request]
 
 permissions:
   contents: read