repoclosure: Use --forcearch for dnf repoclosure
    
    DNF repoclosure requires this option when checking a repository that is
    not compatible with host architecture. It seems that when it is
    compatible, it works as well.
    
    Based on how the list of architectures is generated, we know that the
    main one will always be first.
    
    Fixes: https://pagure.io/pungi/issue/1562
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>