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.

roles/dbscripts/templates/nginx.d.conf.j2

@@ -62,8 +62,9 @@ server {
         proxy_cache_key         $scheme$proxy_host$uri$http_authorization;
 
         # Minimize the number of requests to archweb
-        proxy_cache_lock        on;
-        proxy_cache_use_stale   updating;
+        proxy_cache_lock              on;
+        proxy_cache_use_stale         updating;
+        proxy_cache_background_update on;
 
         # Authentication to archweb
         proxy_pass https://archlinux.org;