These can be quite big so lets cache them. I couldn't find any nots on
ccache in the gitlab docs so I've just ignored it for now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200701135652.1366-36-alex.bennee@linaro.org>