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

pkgs/applications/networking/browsers/chromium/default.nix

@@ -19,9 +19,7 @@ let
   callPackage = newScope chromium;
 
   chromium = {
-    upstream-info = (import ./update.nix {
-      inherit (stdenv) system;
-    }).getChannel channel;
+    upstream-info = (callPackage ./update.nix {}).getChannel channel;
 
     mkChromiumDerivation = callPackage ./common.nix {
       inherit enableSELinux enableNaCl enableHotwording gnomeSupport

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

@@ -1,9 +1,16 @@
-{ system ? builtins.currentSystem }:
+let maybePkgs = import ../../../../../. {}; in
+
+{ stdenv     ? maybePkgs.stdenv
+, runCommand ? maybePkgs.runCommand
+, fetchurl   ? maybePkgs.fetchurl
+, writeText  ? maybePkgs.writeText
+, curl       ? maybePkgs.curl
+, cacert     ? maybePkgs.cacert
+, nix        ? maybePkgs.nix
+}:
 
 let
-  inherit (import ../../../../../. {
-    inherit system;
-  }) lib runCommand fetchurl writeText stdenv curl cacert nix;
+  inherit (stdenv) lib;
 
   sources = if builtins.pathExists ./upstream-info.nix
             then import ./upstream-info.nix