ci: prevent label remove

This prevents the removal of labels that occurs due to a bug in 'actions/labeler' (see actions/labeler#112).

Author: rickstaa

.github/workflows/pr_labeler.yml

@@ -9,3 +9,4 @@ jobs:
       - uses: actions/labeler@v4
         with:
           repo-token: "${{ secrets.GITHUB_TOKEN }}"
+          sync-labels: ''