F40: update osbuild 'distro' value to fedora-40
    
    It's currently set to fedora-39, which I think is why we have
    images that identify themselves as Fedora 39. I don't know to
    what extent they actually have Fedora 39 *content*.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=2264795
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>