mirrorauth: Ensure archweb is queried only once per cache TTL

The first commit adjusts the cache key to not include the original request URI. [1] The second one tweaks how cache entries are populated and refreshed to avoid hitting archweb multiple times while entries are inserted or updated (if stale). [2]

I have tested and deployed these changes. Seems to work well!

[1] this gets us from many auth requests (1 per URL) down to a handful (however many pacman does concurrently)
[2] after this I now only notice at most one auth request per 5 minutes for both nonexistent and stale cache entries

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

@@ -42,7 +42,11 @@ server {
 
         # Cache responses from the auth proxy
         proxy_cache             auth_cache;
-        proxy_cache_key         "$scheme$proxy_host$request_uri$http_authorization";
+        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;
 
         # Authentication to archweb
         proxy_pass https://archlinux.org;