gather: Use fresh cache for each sequence of fus runs
    
    Each depsolved tree will be using its own cache for fus. This should
    still allow for faster loading of metadata after first iteration, but
    should prevent errors from using cached files meant for another variant
    or architecture. The cache is deleted after the last iteration.
    
    JIRA: COMPOSE-3959
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>