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>