Make `notFound()` work in `"use cache"` page (#73210)
When `notFound()` is called in a `"use cache"` function, we need to make
sure that the custom error digest is included in the serialized RSC
payload. Otherwise the `HTTPAccessFallbackErrorBoundary` won't be able
to detect the error as an `HTTPAccessFallbackError`.
`createFlightReactServerErrorHandler` already handles forwarding the
digest correctly for router errors (and other next-internal errors), so
we can just reuse it.
fixes #73130
