ci: check out this project

We can't find out bucket size customizations without checking them out.

.github/workflows/label-pr-size.yml

@@ -10,6 +10,8 @@ jobs:
       contents: write
       pull-requests: write
     steps:
+      - name: Fetch code
+        uses: actions/checkout@v6
       - name: Label and comment PR
         uses: boschresearch/pr-size-labeler@v5
         # Uncomment these lines if you want to use your own custom configuration