workflows/actionlint: enable merge queue/group jobs.

This should allow us to make use of the GitHub merge queue to ensure
that we're not merging outdated code (or breaking `master`) but avoiding
the need to continually merge into/rebase PR branches.

This should be safe to merge as-is as is essentially a no-op without
the merge queue enabled.

Author: MikeMcQuaid

.github/workflows/actionlint.yml

@@ -7,6 +7,7 @@ on:
       - main
       - master
   pull_request:
+  merge_group:
 
 defaults:
   run: