nixos/mailman: increase nginx proxy timeout

In my experience, just after boot, the default timeout of 60 seconds
often isn't quite enough for Mailman.  It's better for the user to
have the request take a little longer than it is to 504.

nixos/modules/services/mail/mailman.nix

@@ -445,6 +445,7 @@ in {
           "${removeSuffix "/" cfg.serve.virtualRoot}/static/".alias = webSettings.STATIC_ROOT + "/";
         };
       });
+      proxyTimeout = mkDefault "120s";
     };
 
     environment.systemPackages = [ (pkgs.buildEnv {