ci: replace actions/checkout with manual git clone for act runner

The act-based Gitea runner executes JS actions inside the specified
container, but ubuntu:22.04 has no Node.js. Use git clone directly
to avoid the dependency.

.gitea/workflows/build.yml

@@ -15,15 +15,17 @@ jobs:
     env:
       DEBIAN_FRONTEND: noninteractive
     steps:
-      - uses: actions/checkout@v4
-
       - name: Install system dependencies
         run: |
           apt-get update
           apt-get install -y \
-            build-essential curl wget pkg-config \
+            build-essential curl wget pkg-config git ca-certificates \
             libgtk-3-dev libwebkit2gtk-4.1-dev libappindicator3-dev librsvg2-dev \
-            libssl-dev git ca-certificates
+            libssl-dev
+
+      - name: Checkout
+        run: |
+          git clone --depth=1 --branch="${GITHUB_REF_NAME}" "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git" .
 
       - name: Install Node.js 22
         run: |