ci: replace actions/checkout with manual git clone (image has no node)

The pazof/yavsc-build-env:debian12-dotnet10-android36-v1 image only
ships .NET 10 SDK + Android SDK + JDK 17, no Node. actions/checkout@v6
requires Node, so the job failed with 'exec: node not found'.

Replace actions/checkout with a direct git clone over HTTPS (Forgejo
anonymous is enabled), and init submodules recursively.

Also drop the bogus docker://image:tag runs-on: matcher, use just 'docker'
to match the runner's declared label name.

.forgejo/workflows/buildAndTest.yml

@@ -40,10 +40,19 @@ jobs:
     runs-on: docker
 
     steps:
-    - uses: actions/checkout@v6
+    - name: Clone yavsc
+      run: |
+        cd "$RUNNER_WORKSPACE"
+        git clone --depth 1 --branch "${GITHUB_REF_NAME:-main}" \
+          https://forgejo.pschneider.fr/notazof/yavsc.git _src
+        cd _src
+        git submodule update --init --recursive --depth 1
     - name: Restore dependencies
+      working-directory: ${{ runner.workspace }}/_src
       run: dotnet restore
     - name: Build
+      working-directory: ${{ runner.workspace }}/_src
       run: dotnet build --no-restore
     - name: Test
+      working-directory: ${{ runner.workspace }}/_src
       run: dotnet test --no-build --verbosity normal