Define and use our own log format for nginx

This is the same as used on luna and as expected by the zabbix nginx monitoring service.

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

@@ -7,7 +7,7 @@ server {
     listen       [::]:80;
     server_name  {{ archwiki_domain }};
 
-    access_log   /var/log/nginx/{{ archwiki_domain }}/access.log;
+    access_log   /var/log/nginx/{{ archwiki_domain }}/access.log main;
     error_log    /var/log/nginx/{{ archwiki_domain }}/error.log;
 
     include snippets/letsencrypt.conf;
@@ -22,7 +22,7 @@ server {
     listen       [::]:443 ssl http2;
     server_name  {{ archwiki_domain }};
 
-    access_log   /var/log/nginx/{{ archwiki_domain }}/access.log;
+    access_log   /var/log/nginx/{{ archwiki_domain }}/access.log main;
     error_log    /var/log/nginx/{{ archwiki_domain }}/error.log;
 
     ssl_certificate      /etc/letsencrypt/live/{{ archwiki_domain }}/fullchain.pem;