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

It is deprecated and will be removed after 18.09.

pkgs/misc/drivers/hplip/default.nix

@@ -36,15 +36,15 @@ let
     "armv7l-linux" = "arm32";
   };
 
-  hplipArch = hplipPlatforms."${stdenv.system}"
-    or (throw "HPLIP not supported on ${stdenv.system}");
+  hplipArch = hplipPlatforms."${stdenv.hostPlatform.system}"
+    or (throw "HPLIP not supported on ${stdenv.hostPlatform.system}");
 
   pluginArches = [ "x86_32" "x86_64" "arm32" ];
 
 in
 
 assert withPlugin -> builtins.elem hplipArch pluginArches
-  || throw "HPLIP plugin not supported on ${stdenv.system}";
+  || throw "HPLIP plugin not supported on ${stdenv.hostPlatform.system}";
 
 pythonPackages.buildPythonApplication {
   inherit name src;