Allow running release notes check in merge queues (#566)

The check will do nothing for merge queues, as we need the PR
information to be able to check anything. But in any case, checking
again is not necessary because PRs should be queued for merge only when
they already pass the release notes check.

Author: llucax

.github/workflows/release_notes_check.yml

@@ -1,6 +1,7 @@
 name: Release Notes Check
 
 on:
+  merge_group:
   pull_request:
     types:
       # On by default if you specify no types.
@@ -11,12 +12,14 @@ on:
       - "labeled"
       - "unlabeled"
 
+
 jobs:
   check-release-notes:
     name: Check release notes are updated
     runs-on: ubuntu-latest
     steps:
       - name: Check for a release notes update
+        if: github.event_name == 'pull_request'
         uses: brettcannon/check-for-changed-files@294a99714e0d350b5083472a293d41bc91804e68 # v1.1.1
         with:
           file-pattern: "RELEASE_NOTES.md"