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

It is deprecated and will be removed after 18.09.

pkgs/development/ruby-modules/testing/stubs.nix

@@ -6,7 +6,7 @@ let
     text = (builtins.toJSON (lib.filterAttrs ( n: v: builtins.any (x: x == n) ["name" "system"]) argSet));
     builder = stdenv.shell;
     args = [ "-c" "echo  $(<$textPath) > $out"];
-    system = stdenv.system;
+    system = stdenv.hostPlatform.system;
     passAsFile = ["text"];
   };
   fetchurl = {url?"", urls ? [],...}: "fetchurl:${if urls == [] then url else builtins.head urls}";