nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/services/networking/pleroma.nix

@@ -75,6 +75,7 @@ in {
         description = "Pleroma user";
         home = cfg.stateDir;
         extraGroups = [ cfg.group ];
+        isSystemUser = true;
       };
       groups."${cfg.group}" = {};
     };