nixos/nix-serve: also set extra-allowed-users

This fixes the case where users enable nix-serve but also have allowed-users set.
Having extra-allowed-users is a no-op when nix.settings.allowed-users is set to "*" (the default)

nixos/modules/services/networking/nix-serve.nix

@@ -67,6 +67,8 @@ in
   };
 
   config = mkIf cfg.enable {
+    nix.settings.extra-allowed-users = [ "nix-serve" ];
+
     systemd.services.nix-serve = {
       description = "nix-serve binary cache server";
       after = [ "network.target" ];