tests/docker: --disable-libssh on ubuntu1804 builds

Currently this stops the mega:

  make docker-test-build

from working. Once the source is patched to deal with the case this
workaround can be removed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/dockerfiles/ubuntu1804.docker

@@ -56,3 +56,6 @@ RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get -y install $PACKAGES
 RUN dpkg -l $PACKAGES | sort > /packages.txt
 ENV FEATURES clang pyyaml sdl2
+
+# https://bugs.launchpad.net/qemu/+bug/1838763
+ENV QEMU_CONFIGURE_OPTS --disable-libssh