Avoid labeling external PRs (#15171)

Doing this to avoid a long-standing bug in GitHub labeler action, which results in failure for the labeler for PRs from forks: https://github.com/actions/labeler/issues/12

.github/workflows/labeler.yml

@@ -7,5 +7,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/labeler@v2
+      if: github.repository == 'aspnet/AspNetCore'
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"