Fix amd64 runner to use Docker mode

Restore runs-on to [self-hosted, linux-amd64] which now maps to Docker
mode after updating the runner configuration. This fixes actions/checkout
compatibility while maintaining amd64 tagging.

🤖 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: ubuntu-latest
+    runs-on: [self-hosted, linux-amd64]
     timeout-minutes: 240
     permissions:
       contents: read