Upgrade sys-apps/pv package from 1.6.6 to 1.6.20

BUG=b:216306532
TEST=built pv on amd64 host

Change-Id: I0a31dc88c71842fc7a831baab0d41d6dda433c01
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3465428
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Gilberto Contreras <gcontreras@google.com>
Commit-Queue: Gilberto Contreras <gcontreras@google.com>