Fix checkout action compatibility with gitea-runner

Downgrade actions/checkout from v4 to v3 to resolve MODULE_NOT_FOUND
errors in gitea-runner's act environment. The v4 action has known
compatibility issues with act's caching mechanism.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/main.yml

@@ -30,7 +30,7 @@ jobs:
       BUILD_CACHE_SCOPE: ws14-docker-linux-server-arm64
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Resolve wylab commit
         id: wylab
@@ -91,7 +91,7 @@ jobs:
       BUILD_CACHE_SCOPE: ws14-docker-linux-server-amd64
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Resolve wylab commit
         id: wylab