.shippable.yml: disable the win cross tests

.shippable.yml: disable the win cross tests

The pkg.mxe.cc package repositories have been down for the last two
weeks causing the builds to fail when shippable re-builds the
containers.

This is really just a sticking plaster until we can get our own docker
hub images properly setup so we can avoid having dependencies on
external repos.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181214151718.5041-1-alex.bennee@linaro.org
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>