Stick tempfiles in /tmp

Rather than putting tempfiles in .. (that is, $CI_BUILDS_DIR) switch to /tmp: this prevents some confusing clobbering that may happen since $CI_BUILDS_DIR is not cleaned across builds and when two jobs end up on the same re-used runner, old files may be incorrectly picked up.

Using /tmp avoids the issue since it is private to the Docker container created for each job.