nixos/etebase-server: set users.users.etebase-server.isSystemUser

- setting users.users.name.{isSystemUser,isNormalUser} is required since
  #115332

nixos/modules/services/misc/etebase-server.nix

@@ -211,6 +211,7 @@ in
 
     users = optionalAttrs (cfg.user == defaultUser) {
       users.${defaultUser} = {
+        isSystemUser = true;
         group = defaultUser;
         home = cfg.dataDir;
       };