It would be faster and less load on the Koji hub if we do not call listMethods before buildContainer.

If a user is running rpkg container-build, they likely know what they are doing, and we don't need to use Look Before You Leap here.

If the buildContainer RPC does not exist, Koji will simply return "GenericError: Invalid method: buildContainer", so we can catch and handle that in the unlikely event that it fails.