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>

roles/robosignatory/templates/robosignatory.toml.j2

@@ -504,7 +504,11 @@ handlers = ["console"]
 
     [consumer_config.coreos]
     bucket = "fcos-builds"
-    key = "{{ (env == 'production')|ternary('fedora-32', 'testkey') }}"
+    {% if env == "staging" %}
+    # Set key to 'testkey' because in staging we only have one key and
+    # detecting which key to use based on FCOS version numbers won't work.
+    key = "testkey"
+    {% endif %}
 
         [consumer_config.coreos.aws]
         access_key = "{{ fcos_builds_releng_aws_access_id }}"