Don't suggest using --arg config '{ allowUnfree = true; }'

This doesn't work when using the Nixpkgs/NixOS channel.

Issue #2998.

pkgs/stdenv/generic/default.nix

@@ -82,9 +82,7 @@ let
               { nixpkgs.config.allowUnfree = true; }
             in configuration.nix to override this. If you use Nix standalone, you can add
               { allowUnfree = true; }
-            to ~/.nixpkgs/config.nix or pass
-              --arg config '{ allowUnfree = true; }'
-            on the command line.''
+            to ~/.nixpkgs/config.nix.''
         else if !allowBroken && attrs.meta.broken or false then
           throw "you can't use package ‘${attrs.name}’ in ${pos'} because it has been marked as broken"
         else if !allowBroken && attrs.meta.platforms or null != null && !lib.lists.elem result.system attrs.meta.platforms then