Skip freeze check in the merge queue (flutter#184854)

`merge_group` events do not have the same GitHub API access as
`pull_request_target`, so we should skip the check in the merge queue.
We should skip the freeze check in the merge queue. The freeze workflow
was enabled as a required check in the branch protections in order to
capture existing PRs and prevent them from landing. This protection made
the check required in the merge queue, but no way for it to handle it.
After this lands, I can re-enable the branch protection.

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Author: Piinks

.github/workflows/freeze.yml

@@ -9,6 +9,9 @@ on:
     types: [opened, reopened, synchronize, ready_for_review, labeled, unlabeled]
     branches:
       - master
+  merge_group:
+    branches:
+      - master
 
 permissions: read-all
 
@@ -20,6 +23,7 @@ jobs:
 
     steps:
       - name: Check for changes in frozen folders
+        if: github.event_name != 'merge_group'
         uses: dorny/paths-filter@v3
         id: filter
         with:
@@ -40,7 +44,7 @@ jobs:
               - 'packages/flutter/test_fixes/cupertino/**'
 
       - name: Fail on frozen changes
-        if: steps.filter.outputs.frozen == 'true' && !contains(github.event.pull_request.labels.*.name, 'override code freeze')
+        if: github.event_name != 'merge_group' && steps.filter.outputs.frozen == 'true' && !contains(github.event.pull_request.labels.*.name, 'override code freeze')
         run: |
           echo "Error: Code changes detected during the current code freeze."
           echo "If this is a critical fix that must land, please file an issue for team-design."