mailman: Ensure the public archives aren't cached by the browser

It confuses the users that the browser is caching them (due to
heuristic[1]).

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#heuristic_freshness_checking

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

@@ -39,6 +39,7 @@ server {
 
     location /pipermail {
         alias /var/lib/mailman/archives/public;
+        add_header Cache-Control "public, no-cache";
         autoindex on;
     }