nixos/npm: add `npm` to the system closure by default (#42420)

nixos/modules/programs/npm.nix

···
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
···
     environment.etc."npmrc".text = cfg.npmrc;
 
     environment.variables.NPM_CONFIG_GLOBALCONFIG = "/etc/npmrc";
+
+    environment.systemPackages = [ pkgs.nodePackages.npm ];
   };
 
 }