fix: use correct platform when creating remote buildx builder

The remote builder was hardcoded to use --platform linux/amd64
regardless of user input or runner architecture. This caused
performance issues on ARM runners and cache inefficiencies.

Now properly uses the platforms input or detects host architecture
to avoid unnecessary QEMU emulation and improve build performance.

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

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

src/__mocks__/execa.ts

@@ -4,4 +4,4 @@ export const execa = jest.fn().mockImplementation(() => {
     stderr: '',
     exitCode: 0
   };
-}); 
+});