Ref actions by commit SHA in labeler.yml

It's important to make sure the SHA's are from the original repositories and not forks.

For reference:

https://github.com/actions/labeler/releases/tag/v4.1.0
9fcb2c2f55


Signed-off-by: Gabriela Gutierrez <gabigutierrez@google.com>

.github/workflows/labeler.yml

@@ -12,6 +12,6 @@ jobs:
       pull-requests: write  # for actions/labeler to add labels to PRs
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/labeler@v4
+    - uses: actions/labeler@9fcb2c2f5584144ca754f8bfe8c6f81e77753375 # v4.1.0
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"