Handle new checksum format for CoreOS signing requests
    
    In order to be more self-documenting and forward-compatible, the
    checksum field for signing requests is now prefixed by the algorithm
    used to derive it. Handle this new format.
    
    See https://github.com/coreos/fedora-coreos-tracker/issues/198#issuecomment-533683246
    
    Signed-off-by: Jonathan Lebon <jonathan@jlebon.com>