Fix CI: replace actions/checkout with manual git clone

The act runner executes actions/checkout inside the job container,
but that action is a Node.js script and rust:1.86-slim has no node.
Use plain git clone instead.

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

.gitea/workflows/ci.yml

@@ -11,19 +11,29 @@ env:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs_on: ubuntu-latest
     container:
       image: rust:1.86-slim
     steps:
-      - uses: actions/checkout@v4
+      - name: Install git
+        run: apt-get update && apt-get install -y --no-install-recommends git
+
+      - name: Checkout
+        run: |
+          git config --global --add safe.directory '*'
+          git clone --depth 1 --branch ${GITHUB_REF_NAME:-main} ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git .
+        working_directory: /workspace
 
       - name: Run tests
         run: cargo test -- --test-threads=1
+        working_directory: /workspace
 
       - name: Build release
         run: cargo build --release
+        working_directory: /workspace
 
       - name: Check binary
         run: |
           ls -lh target/release/btest
           target/release/btest --version
+        working_directory: /workspace