Return proper headers for 404s in review apps

Why is this change being made?

When we got a 404 from the origin in review apps, we were fetching the 404 page but returning the headers for that last request instead of the original request.

This issue only affects review apps. New production (cf.about.gitlab.com) would have been working fine as that uses c.storage.googleapis.com, which returns 404 headers and the 404 page content as the body so there is no extra request.

Reported in https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/18945#note_1450196281.

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