Skip postinstall in image_to_usb.sh --install

It's not appropriate to run the device postinstall on the build
machine, so pass --skip_postinstall to chromeos-isntall.

BUG=chromium:217621
TEST=image_to_usb.sh --install

Change-Id: I1076849430caed4467b31188003181155ac83f6a
Reviewed-on: https://gerrit.chromium.org/gerrit/58425
Reviewed-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Tested-by: Michael Spang <spang@chromium.org>
1 file changed