Remove references to <nixpkgs>

Nixpkgs should not use <nixpkgs> to refer to itself, because $NIX_PATH
might not be set or not point to the same Nixpkgs. It's also
unnecessary.

pkgs/tools/networking/airfield/default.nix

@@ -3,7 +3,7 @@
 with stdenv.lib;
 
 let 
-  nodePackages = callPackage (import <nixpkgs/pkgs/top-level/node-packages.nix>) {
+  nodePackages = callPackage (import ../../../top-level/node-packages.nix) {
     neededNatives = [python] ++ optional (stdenv.isLinux) utillinux;
     self = nodePackages;
     generated = ./package.nix;