In commit 6957fd98 ("gitlab: add avocado asset caching") we
tried to save the Avocado cache (as in commit c1073e44 with
Travis-CI) however it doesn't work as expected. For some reason
Avocado uses /root/avocado_cache/ which we can not select later.

Manually generate a Avocado config to force the use of the
current job's directory.

This patch is based on an earlier version from Philippe Mathieu-Daudé.

Message-Id: <20200730141326.8260-5-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>