updates default.nix to use argument passing.

default.nix

 {
-  pkgs ? import <argopkgs> {},
+  pkgs ? import <argopkgs> {argotk-src=./.;},
 }:
 rec {
-  argotk = pkgs.argotk.overrideAttrs (old:{
-    src = ./.;
-  });
-
+  argotk = pkgs.argotk;
   hack = pkgs.argolib.getHackEnv pkgs pkgs.haskellPackages argotk;
-
   shakeEnv = pkgs.stdenv.mkDerivation {
     name = "shake";
     ARGOTK_SHARE = "${argotk}/share";