Due to a design problem and misunderstanding between the Avocado
framework and QEMU, Avocado is fetching many asset artifacts it
shouldn't be fetching, exhausting the jobs CI timeout.
Since Avocado artifacts are cached, this is not an issue with old
forks, which already have populated the cache and do not need to
download new artifacts to run the tests.
However this is very confusing to new contributors who start to
fork the project and keep having failing CI pipelines.
As a temporary kludge, add the QEMU_CI_AVOCADO_TESTING variable
to allow old forks to keep running the Avocado tests, while still
allowing new forks to use the mainstream set of CI tests.