reewide: Purge all uses stdenv.system and top-level system

It is deprecated and will be removed after 18.09.

pkgs/development/libraries/physics/geant4/fetch.nix

@@ -1,7 +1,7 @@
 { stdenv, fetchurl }:
 
 let
-  fetch = { version, src ? builtins.getAttr stdenv.system sources, sources ? null }:
+  fetch = { version, src ? builtins.getAttr stdenv.hostPlatform.system sources, sources ? null }:
   {
     inherit version src;
   };