build_packages: log PORTAGE_BINHOST to make it easy to diagnose prebuilt issues on ci builders

BUG=chromium:947238
TEST=`./build_packages --board=eve` shows the log

Change-Id: I7c267bdc41e5ccf1077efc6cc629ec8971bedaac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1553467
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Tested-by: Ned Nguyen <nednguyen@google.com>
1 file changed