chromium: take gnChromium from buildPackages

${gnChromium}/bin/gn needs to be compiled for the buildPlatform
since it is run at build time.

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

@@ -1,4 +1,5 @@
 { newScope, config, stdenv, fetchurl, makeWrapper
+, buildPackages
 , llvmPackages_16
 , ed, gnugrep, coreutils, xdg-utils
 , glib, gtk3, gtk4, gnome, gsettings-desktop-schemas, gn, fetchgit
@@ -47,7 +48,7 @@ let
       inherit channel chromiumVersionAtLeast versionRange;
       inherit proprietaryCodecs
               cupsSupport pulseSupport ungoogled;
-      gnChromium = gn.overrideAttrs (oldAttrs: {
+      gnChromium = buildPackages.gn.overrideAttrs (oldAttrs: {
         inherit (upstream-info.deps.gn) version;
         src = fetchgit {
           inherit (upstream-info.deps.gn) url rev sha256;