Path to the React Router Vite server build (default export = ServerBuild).

Must be absolute — an absolute filesystem path or a file: URL. The default loader performs await import(serverBuildPath), and a relative specifier there resolves against this plugin's own module rather than against the consuming application, so it can never find the build. From an application module, derive one with new URL('./build/server/index.js', import.meta.url).href.