fix: CI use actions/checkout@v4 (Gitea proxies to GitHub)

The full URL form was 404. The short form lets Gitea resolve from
its configured action sources (GitHub proxy). This worked for build #7.

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

.gitea/workflows/build-iso.yml

@@ -11,19 +11,9 @@ jobs:
     timeout-minutes: 30
     steps:
       - name: Checkout
-        run: |
-          cd /home/archipelago
-          WORKSPACE="${GITHUB_WORKSPACE}"
-          CLONE_URL="${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git"
-          if [ -n "$GITHUB_TOKEN" ]; then
-            CLONE_URL=$(echo "$CLONE_URL" | sed "s|https://|https://token:${GITHUB_TOKEN}@|")
-          fi
-          [ -d "$WORKSPACE" ] && find "$WORKSPACE" -mindepth 1 -delete
-          git clone --depth 1 --branch "${GITHUB_REF_NAME:-main}" "$CLONE_URL" "$WORKSPACE"
-          cd "$WORKSPACE"
-          echo "Checked out $(git rev-parse --short HEAD) on ${GITHUB_REF_NAME:-main}"
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
 
       - name: Build backend
         run: |