enter_chroot: also check SUDO_USER

Start with SUDO_USER before falling back to USER in case we're being
run through sudo.

BUG=chromium:1025578
TEST=CQ passes

Change-Id: Ic33aef2e870c98e4450aa9eb0c9eb6e43d647a51
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1967191
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed