Keep all acceptance-test assets in the same cache bucket.

As of v5.0.0-rc1, the cache is 2610.11MB:
https://travis-ci.org/github/philmd/qemu/jobs/670558103



Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200403172919.24621-6-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>