build_image: disable network access

There's no need for build_image to access the network as it has all
the files already downloaded, so enforce it by turning off access.

BUG=chromium:1068391
TEST=CQ passes

Change-Id: I0da4d96635464f7dd0daf05c2e26d359188476b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2137982
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed