pages router: ensure x-middleware-cache is respected (#67734)
`x-middleware-cache: "no-store"` in Pages router is a way to signal to
the client that it should not store the response in the cache. However
in certain circumstances, namely when `unstable_skipClientCache` is
true, the data request would be awaited and then stored in the
`inflightCache` regardless of the header.
The original implementation of this in the router has logic to delete
the response from the inflight cache after the request has fulfilled
because `inflightCache` stores the unresolved promise. But in this
optimistic prefetch case, when we're only storing it in the cache once
the request is fulfilled, we can prevent a race condition where the
ignored prefetch is erroneously re-added to the cache by ensuring it's
never added to the cache to begin with if the response says not to.
Fixes #66881
Closes NEXT-3550
