nixos/tests/chromium: Propagate "system" to pkgs

Assigning the channelMap by the function attrset argument at the
top-level of the test expression file may reference a different
architecture than we need for the tests.

So if we get the pkgs attribute by auto-calling, this will lead to test
failure because we have a different architecture for the test than for
the browser.

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