chore: gate label-prs to pull requests (#2892)

Signed-off-by: Patrick Knight <pknight@redhat.com>

Author: PatAKnight

.github/workflows/add-workspace-label.yml

@@ -31,6 +31,7 @@ jobs:
           repo-token: ${{ secrets.GITHUB_TOKEN }}
 
   label-prs:
+    if: github.event_name == 'pull_request_target'
     runs-on: ubuntu-latest
     permissions:
       pull-requests: write