fix: metadata image route normalize path posix for windows (vercel#71673

)

### Why?

When handling the opengraph-image size exceeding the limit, we mapped
with string literal between Rust and JavaScript. The Rust string literal
contains backslashes `\` on Windows, incompatible with JavaScript string
syntax.

Following up on vercel#71615.

Fixes vercel#71582

packages/next/src/build/webpack/loaders/next-metadata-route-loader.ts

@@ -76,6 +76,8 @@ async function getStaticAssetRouteCode(
   resourcePath: string,
   fileBaseName: string
 ) {
+  resourcePath = path.posix.normalize(resourcePath)
+
   const cache =
     fileBaseName === 'favicon'
       ? 'public, max-age=0, must-revalidate'