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>
diff --git a/image_to_usb.sh b/image_to_usb.sh
index 8040397..a513b81 100755
--- a/image_to_usb.sh
+++ b/image_to_usb.sh
@@ -360,7 +360,8 @@
       --skip_dst_removable \
       --arch="${FLAGS_arch}" \
       --payload_image="${SRC_IMAGE}" \
-      --dst="${FLAGS_to}"
+      --dst="${FLAGS_to}" \
+      --skip_postinstall
   fi
 elif [[ "${FLAGS_to}" == /dev/* ]]; then
   # Did the user attempt to write to a non-existent block device?