Multiple instances of fus running at the same time use wrong cache

[lubomir]

When there are two instances of fus spawned at about the same time and use same repo names for different repos, it can cause one of the processes to not be able to download repo metadata.

As a workaround in Pungi, I'm setting XDG_CACHE_HOME to use a fresh cache for sequence of fus runs that all use the same repos. This cache is deleted afterwards.