Update repo-freeze-check.yml (#18014)

Co-authored-by: Chiedo John <2156688+chiedo@users.noreply.github.com>

Author: heiskr

.github/workflows/repo-freeze-check.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Fail if repo merges are paused
-        if: ${{ env.FREEZE == 'true' && github.ref != 'refs/heads/repo-sync' }}
+        if: ${{ env.FREEZE == 'true' && github.head_ref != 'repo-sync' }}
         run: |
           echo 'Merges into the "main" branch on this repo are currently paused!'
           exit 1