ELN: Allow reusing old Pungi gather phase results.
    
    Pungi-4.2.21 has new performance optimization - it can reuse
    gather phase results from previous compose runs in certain
    situations:
    
    https://pagure.io/pungi/pull-request/1360
    
    This commit enables this for ELN composes.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>