nixos/harmonia: set extra-allowed-users

This fixes the case where users enable harmonia 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/harmonia.nix

@@ -28,6 +28,8 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    nix.settings.extra-allowed-users = [ "harmonia" ];
+
     systemd.services.harmonia = {
       description = "harmonia binary cache service";