chromium/updater: Don't import <nixpkgs> again

This effectively resets the attributes given at the point the main
<nixpkgs> is imported and thus for example is also reading in stuff like
~/.nixpkgs/config.nix again, which might lead to unexpected results.

We now only import <nixpkgs> now if the updater is auto-called (like in
update.sh), otherwise the required attributes are passed by callPackage
within the Chromium scope.

I remember noting about this a while ago either on IRC or on GitHub, but
I can't find it right now, so thanks to @obadz for reminding me about
this in #15225.

Tested this by running the updater and also using:

NIXPKGS_CONFIG=$(pwd)/broken.nix nix-instantiate --arg config {} -A chromium

The contents of broken.nix were:

EVALERR{

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fixes: #15225