Update check-restricted-files.yaml

Author: ralfhandl

.github/workflows/check-restricted-files.yaml

@@ -14,6 +14,8 @@ jobs:
   check-files:
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v5 # checkout repo content
+
       - name: Check changed files
         shell: bash
         run: |