Update pull_request_label.yml (flutter#3647)

Author: Chris Yang

.github/workflows/pull_request_label.yml

@@ -16,7 +16,7 @@ jobs:
   label:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/labeler@v3
+      - uses: actions/labeler@9794b1493b6f1fa7b006c5f8635a19c76c98be95
         with:
           repo-token: "${{ secrets.GITHUB_TOKEN }}"
           sync-labels: true
@@ -25,7 +25,7 @@ jobs:
     if: github.event.action == 'closed' && github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/labeler@v3
+      - uses: actions/labeler@9794b1493b6f1fa7b006c5f8635a19c76c98be95
         with:
           repo-token: "${{ secrets.GITHUB_TOKEN }}"
           configuration-path: .github/post_merge_labeler.yml