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>
chromium: Fix userns sandbox patch for version 36.

This fixes build for version 36, which i accidentally broke in commit
f6e31fadd80486cdd68e3ad3d7ae888aa81400b9.

The reason this happened, was that my Hydra didn't pick up the latest
commit and I actually tested and built the parent commit instead of the
update commit.

So, this commit is the real "builds fine, tested" for all channels.

Also, the sandbox client initalization has moved into
setuid_sandbox_client.cc, so we need to move the lookup of the
CHROMIUM_SANDBOX_BINARY_PATH environment variable there.

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