CI - Label check always runs (#4594)

# Description of Changes

The CI check for the Do Not Merge label was only running on label-change
events, so if a PR never received a label (any label), the check would
never run. This meant that the check couldn't be marked required because
it was missing on PRs that were never labeled anything.

Additionally, the check was not configured to run properly in the merge
queue, so it would have blocked the merge queue if it were a required
check.

This PR fixes those issues by making the check run on more pull request
events, and by trivially succeeding in the merge queue.

This enables the check to be made required.

# API and ABI breaking changes

None. CI only.

# Expected complexity level and risk

1

# Testing
- [x] The check is now showing as run on this PR
- [x] The check still fails if I add a "do not merge" label

---------

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

.github/workflows/check-merge-labels.yml

@@ -2,17 +2,21 @@ name: Check merge labels
 
 on:
   pull_request:
-    types: [labeled, unlabeled]
+    types: [opened, reopened, labeled, unlabeled]
   merge_group:
+
 permissions: read-all
 
 jobs:
   label_checks:
+    name: Check merge labels
     runs-on: ubuntu-latest
     steps:
-      - id: manually_blocked
-        if: |
-          contains(github.event.pull_request.labels.*.name, 'do not merge')
+      - if: github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'do not merge')
         run: |
           echo "This is labeled \"Do not merge\"."
           exit 1
+
+      # If we're in a merge queue, the PR has already passed checks these checks before being added to the queue.
+      - if: github.event_name == 'merge_group'
+        run: echo "Merge group run; skipping merge-label checks."