Silverblue/Kinoite/Sericea/Onyx: Use unified core mode
    
    rpm-ostree is moving to unified core composes and this is now working
    for Silverblue, Kinoite, Sericea and Onyx.
    
    See: https://github.com/coreos/rpm-ostree/issues/729
    See: https://fedoraproject.org/wiki/Changes/FedoraSilverblueUnifiedCore
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2150984
    
    Related changes:
    - https://pagure.io/workstation-ostree-config/pull-request/246
    - https://pagure.io/workstation-ostree-config/pull-request/344
    
    Changes in Pungi:
    - https://pagure.io/pungi/pull-request/1626
    - https://pagure.io/pungi/pull-request/1690
    
    For previous attempts, see:
    - https://pagure.io/pungi-fedora/pull-request/1115
    - https://pagure.io/pungi-fedora/pull-request/1136 (revert)
    - https://pagure.io/pungi-fedora/pull-request/1145
    - https://pagure.io/pungi-fedora/pull-request/1185 (revert)
    
    Signed-off-by: Timothée Ravier tim@siosm.fr