presubmits: respect CHROMEOS_ROOT_DIRECTORY when entering the chroot

BUG=chromium:1100659
TEST=Ran presubmits inside of the CrOS tree, and outside with
     CHROMEOS_ROOT_DIRECTORY set.

Change-Id: I119d13f8f7b6b0f8a1c94750fe5e90be5ad952be
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2274728
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed