Merge pull request #119698 from flokli/nullmailer-is-system-user

nixos/nullmailer: set isSystemUser

nixos/modules/services/mail/nullmailer.nix

···
       users.${cfg.user} = {
         description = "Nullmailer relay-only mta user";
         group = cfg.group;
+        isSystemUser = true;
       };
 
       groups.${cfg.group} = { };