chromium/source: Propagate system attribute.

The system attribute was already there in the function head of the
shared update helper but it actually wasn't used and thus later the
import of <nixpkgs> was done using builtins.currentSystem instead of the
system attribute inherited from the source derivation.

Now we correctly propagate the attribute, so that even when running a
64bit kernel you can run a 32bit Chromium with binary plugins.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/networking/browsers/chromium/source/update.nix

@@ -1,7 +1,9 @@
 { system ? builtins.currentSystem }:
 
 let
-  inherit (import <nixpkgs> {}) lib writeText stdenv;
+  inherit (import <nixpkgs> {
+    inherit system;
+  }) lib writeText stdenv;
 
   sources = if builtins.pathExists ./sources.nix
             then import ./sources.nix