turbopack: add vary header to app routes (#47946)
Next.js adds a `Vary: RSC, Next-Router-State-Tree, Next-Router-Prefetch`
to app routes for both rendering and flight requests. Without this, the
browser can mistakenly serve the flight request to the app render, and
vice versa.
This only adds `Vary: RSC` for the moment, I'm not sure what the others
are used for. We also don't add a `Cache-Control: max-age=0` header, but
I'm not sure if that's necessary either.
Fixes NEXT-657
fix #45595
