fix: CI checkout uses manual git clone instead of missing action

The actions/checkout@v4 action was 404 on git.tx1138.com causing
instant build failures. Use manual git clone for reliability with
host-mode runner.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-iso.yml

@@ -11,9 +11,17 @@ jobs:
     timeout-minutes: 30
     steps:
       - name: Checkout
-        uses: https://git.tx1138.com/actions/checkout@v4
-        with:
-          fetch-depth: 1
+        run: |
+          WORKSPACE="${GITHUB_WORKSPACE:-$(pwd)}"
+          cd "$WORKSPACE"
+          if [ -d .git ]; then
+            git fetch origin "${GITHUB_REF_NAME:-main}" --depth 1
+            git checkout FETCH_HEAD
+          else
+            git clone --depth 1 --branch "${GITHUB_REF_NAME:-main}" \
+              "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git" "$WORKSPACE"
+          fi
+          echo "Checked out $(git rev-parse --short HEAD) on ${GITHUB_REF_NAME:-main}"
 
       - name: Build backend
         run: |