coreos: Detect what key to use based on build id
    
    Our Fedora CoreOS version numbers are predictable in that they
    encode the Fedora major version into the first 2 digits of the
    version; i.e, `31.20200505.3.0`. Let's auto detect what key to
    use based on that build id.
    
    Fixes: https://github.com/coreos/fedora-coreos-tracker/issues/296
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>