ci(label): use head_ref for REF

.github/workflows/label.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/github-script@v6
         env:
           LABEL: ${{ github.event.label.name }}
-          REF: ${{ github.ref_name }}
+          REF: ${{ github.head_ref || github.ref_name }}
         with:
           script: |
             const { LABEL, REF, HEAD_SHA } = process.env;