Never create a predictable dir in /tmp for caching
Never create a predictable dir in /tmp for caching

Users may run appstreamcli or any other AppStream-using tool as root and
thereby accidentally create
/tmp/appstream, which is a predictable name and may be exploited by
malicious actors. With this patch, we either write to /root/.cache, or,
in case that is not possible, create a temporary directory with a random
name in /tmp.

ximion/appstream,ximion/appstream,ximion/appstream,ximion/appstream