CI: use git-based checkout instead of actions/checkout

.forgejo/workflows/ci.yml

@@ -10,6 +10,11 @@ jobs:
     container:
       image: python:3.12-slim
     steps:
-      - uses: actions/checkout@v4
+      - name: Checkout
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          command -v git >/dev/null || (apt-get update -qq && apt-get install -y -qq git)
+          git clone --depth 1 --branch "$GITHUB_REF_NAME" "https://x-access-token:${GITHUB_TOKEN}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git" .
       - run: pip install --no-cache-dir -r requirements.txt
       - run: python -m py_compile app.py