Fix PR labeler permissions

Although the permission to modify PRs is granted to the entire workflow, the job still reports that it does not the permission to do so:
GITHUB_TOKEN Permissions
  Contents: read
  Metadata: read
  PullRequests: read
This adds the permission to the job directly

.github/workflows/pr-labeler.yml

@@ -8,6 +8,8 @@ jobs:
   changed-lines-count-labeler:
     runs-on: ubuntu-latest
     name: Automatically labelling pull requests based on the changed lines count
+    permissions:
+      pull-requests: write
     steps:
     - name: Set a label
       uses: vkirilichev/changed-lines-count-labeler@v0.2