ci: use manual git checkout in Arch container

actions/checkout requires Node.js, which isn't in archlinux:latest.
Manual git clone + checkout FETCH_HEAD instead. Adds git to pacman.

.gitea/workflows/ci.yml

@@ -11,12 +11,16 @@ jobs:
     runs-on: ubuntu-latest
     container: archlinux:latest
     steps:
-      - uses: actions/checkout@v4
-
       - name: Install dependencies
         run: |
           pacman -Syu --noconfirm
-          pacman -S --noconfirm clang cmake ninja
+          pacman -S --noconfirm clang cmake ninja git
+
+      - name: Checkout
+        run: |
+          git clone --depth 1 "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY" .
+          git fetch origin "$GITHUB_REF"
+          git checkout FETCH_HEAD
 
       - name: Build kappa
         run: |