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/web-apps/nextcloud.nix

@@ -607,6 +607,7 @@ in {
         home = "${cfg.home}";
         group = "nextcloud";
         createHome = true;
+        isSystemUser = true;
       };
       users.groups.nextcloud.members = [ "nextcloud" config.services.nginx.user ];