GH Actions: safeguard the PR label workflow some more

.github/workflows/label-new-prs.yml

@@ -2,16 +2,48 @@ name: Label new PRs
 
 on:
   # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target
+  # The `pull_request_target` event is used for "normal" PRs to label them when they are opened.
+  # This will use the `labeler.yml` file in the default (master) branch of the repo.
   pull_request_target:
     types:
       - opened
       - ready_for_review
 
+  # The `pull_request` event is used for PRs which change the files which handle the labeling to prevent a silently failing action.
+  # This will use the `labeler.yml` file in the PR branch.
+  pull_request:
+    paths:
+      - '.github/workflows/label-new-prs.yml'
+      - '.github/labeler.yml'
+
 jobs:
   label-new-prs:
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'PHPCSStandards' && github.event_name == 'pull_request_target'
+
+    name: Add labels to new PRs
+
+    steps:
+      - name: Label new PRs
+        uses: srvaroa/labeler@master
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+
+  validate-labeler-worflow:
+    runs-on: ubuntu-latest
+    if: github.repository_owner == 'PHPCSStandards' && github.event_name == 'pull_request' && github.event.pull_request.merged == false
+
+    name: Validate changes to Labeler logic
 
     steps:
-      - uses: srvaroa/labeler@master
+      # Checkout is needed to use the `use_local_config` option.
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Verify changes to the labeling logic
+        uses: srvaroa/labeler@master
+        with:
+          use_local_config: true
+          fail_on_error: true
         env:
           GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"