gitlab-ci.yml: Set artifacts expiration time

gitlab-ci.yml: Set artifacts expiration time

The default expiration time for artifacts seems to be very high (30 days?).
Since we only need the artifacts to pass the binaries from one stage to
the next one, we can decrease the expiration time to avoid to spam the
file server too much. Two days should be enough in case someone still wants
to have a look after the pipeline finished.

Message-Id: <20200806161546.15325-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>