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

It is deprecated and will be removed after 18.09.

pkgs/applications/networking/dropbox/default.nix

@@ -2,7 +2,7 @@
 
 let platforms = [ "i686-linux" "x86_64-linux" ]; in
 
-assert lib.elem stdenv.system platforms;
+assert lib.elem stdenv.hostPlatform.system platforms;
 
 # Dropbox client to bootstrap installation.
 # The client is self-updating, so the actual version may be newer.
@@ -12,7 +12,7 @@ let
   arch = {
     "x86_64-linux" = "x86_64";
     "i686-linux"   = "x86";
-  }.${stdenv.system};
+  }.${stdenv.hostPlatform.system};
 
   installer = "https://clientupdates.dropboxstatic.com/dbx-releng/client/dropbox-lnx.${arch}-${version}.tar.gz";
 in