A recent change to the EC2 plugin is incompatible with CasC setup for it.  Specifically, the `EC2 Key Pair's Private Key` field has changed from a text entry into a select list from a credentials provider. 

The EC2 provider is expecting secrets of type "SSH Username with Private key" but the AWS SecretsManager credential provider only supports "SSH User private key".

The net result is that I can launch runner nodes because I can't access my private keys in AWS SecretsManager, and the old text entry format is gone.