ci: add checkout step to labeler workflow and use pull_request_target

actions/labeler@v6 falls back to the API when the config file is not found
locally. Adding actions/checkout ensures .github/labeler.yml is available
in the workspace, removing the API fallback.

Switching the trigger from pull_request to pull_request_target gives
GITHUB_TOKEN write access on pull-requests for fork PRs, fixing the
"Resource not accessible by integration" error that blocked label assignment.

Author: MarTrepodi

.github/workflows/labeler.yml

@@ -1,7 +1,7 @@
 name: Auto Label PR
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, synchronize]
 
 permissions:
@@ -13,6 +13,7 @@ jobs:
     name: Auto Label
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: actions/labeler@v6
         with:
           repo-token: ${{ secrets.GITHUB_TOKEN }}