Prevent caching page with 304 status (#57737)
I think that sometimes when a revalidation happens from a request with
caching headers this causing the 304 status to be cached.
This PR ensures the 304 from an initial response doesn't affect a
background revalidation.
Fixes: https://github.com/vercel/next.js/issues/56580
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
