cros_workon: switch to cros_list_overlays

Newer portage warns when you try to query PORTDIR_OVERLAY, so switch to
our own cros_list_overlays to get the same information.

BUG=chromium:398269
TEST=`cros_workon-link --all list` output is unchanged
TEST=`cros_workon-link --all list --board link` output is unchanged
CQ-DEPEND=CL:226735

Change-Id: I23fbcf1033ddc11be9d8c9aad9fd29f654057ef5
Reviewed-on: https://chromium-review.googlesource.com/226736
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed