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/build-support/vm/windows/cygwin-iso/default.nix

@@ -40,7 +40,7 @@ let
     };
   in map gen expr;
 
-in import <nixpkgs/nixos/lib/make-iso9660-image.nix> {
+in import ../../../../../nixos/lib/make-iso9660-image.nix {
   inherit stdenv perl cdrkit pathsFromGraph;
   contents = [
     { source = "${cygwinCross}/bin/setup.exe";