Use /var/tmp for nginx cache path

The /var/tmp directory is not mounted as tmpfs unlike /tmp which is
mounted as such on some distros like Fedora or Arch. Since there isn't
really a benefit to having the cache on tmpfs, this change should allow
for a larger cache if needed without worrying about running out of RAM.

installation/nginx/akkoma.nginx

@@ -3,7 +3,7 @@
 # See the documentation at docs.akkoma.dev for your particular distro/OS for
 # installation instructions.
 
-proxy_cache_path /tmp/akkoma-media-cache levels=1:2 keys_zone=akkoma_media_cache:10m max_size=1g
+proxy_cache_path /var/tmp/akkoma-media-cache levels=1:2 keys_zone=akkoma_media_cache:10m max_size=1g
                  inactive=720m use_temp_path=off;
 
 # this is explicitly IPv4 since Pleroma.Web.Endpoint binds on IPv4 only