nixos-generate-config: add 'lib' arg to configuration.nix

Although 'lib' isn't needed by the default configuration, new NixOS uses
can get confused when being recommended configuration.nix snippets like

  systemd.services.NetworkManager-wait-online.enable = lib.mkForce false;

and have `sudo nixos-rebuild switch` fail with

  error: undefined variable 'lib'

nixos/modules/installer/tools/tools.nix

@@ -126,7 +126,7 @@ in
       # your system.  Help is available in the configuration.nix(5) man page
       # and in the NixOS manual (accessible by running `nixos-help`).
 
-      { config, pkgs, ... }:
+      { config, lib, pkgs, ... }:
 
       {
         imports =