update_chroot: Also update target-sdk-post-cross

Add virtual/target-sdk-post-cross to the list of packages updated by
update_chroot.  This ensures that packages like rust and cargo are
properly updated.

BUG=chromium:820330
TEST=./update_chroot updates rust to 1.24

Change-Id: I5604b91a57b6a3ca4207e6cb1d6dccb6b6d45ad0
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/957936
Reviewed-by: Zach Reizner <zachr@chromium.org>
1 file changed