dbscripts: Enable proxy_cache_background_update for mirrorauth

This allows serving a stale response even to the request that triggers an update. This should ensure all requests finish quickly.

With just proxy_cache_use_stale updating, the request that attempts to update the cache waits for the response, while all other requests get to use the stale response.

Currently archweb is badly overloaded and can take over half a minute to respond. Pacman is not that patient and fails the download.