robosignatory: coreos: only set key in config for staging
    
    Upstream robosignatory was changed to auto-detect the key to use
    for signing for Fedora CoreOS builds in the following PR:
    https://pagure.io/robosignatory/pull-request/46#
    
    Let's switch to only setting the key in the config for the staging
    environment so that we can start using the auto-detection.
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>