host_vars: add fastcgi_cache to apollo

The wiki requires an nginx cache for mediawiki's static assets which are
served through PHP causing an exhaustion of php-fpm workers. Caching in
nginx is a good way to circumvent this.

host_vars/apollo.archlinux.org

@@ -43,3 +43,5 @@ fail2ban_jails:
   postfix: true
   dovecot: false
   nginx: true
+
+fastcgi_cache: wiki