We already are in the <nixpkgs> repository and using <nixpkgs> within
its own tree will depend on <nixpkgs> to be set in NIX_PATH, otherwise
the evaluation will fail.
So apart from this issue it isn't even necessary to use <nixpkgs>
instead of the relative path to node-packages.nix, so let's use the
latter.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @uwap, @Mic92