Switch amd64 job to Docker runner mode

Change build-amd64 from host mode to ubuntu-latest Docker runner to
fix actions/checkout compatibility. The checkout action requires a
proper containerized environment that host mode doesn't provide.

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

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

.github/workflows/main.yml

@@ -81,7 +81,7 @@ jobs:
 
   build-amd64:
     # Requires a self-hosted Linux amd64 runner
-    runs-on: [self-hosted, linux-amd64]
+    runs-on: ubuntu-latest
     timeout-minutes: 240
     permissions:
       contents: read