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>