CI - Label check runs on synchronize events (#4602)

# Description of Changes

This shouldn't actually be required in the long-term, because it runs on
new PRs and any label changes, but the case where it _is_ required? PRs
that are currently already open and merge in `master`. That triggers a
`synchronize` event but not any of the events that this check runs on.
Sigh.

# API and ABI breaking changes

None

# Expected complexity level and risk

1

# Testing

None

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

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

@@ -2,7 +2,7 @@ name: Check merge labels
 
 on:
   pull_request:
-    types: [opened, reopened, labeled, unlabeled]
+    types: [opened, reopened, synchronize, labeled, unlabeled]
   merge_group:
 
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