s390x: failed to generate asset "Image": multiple "disk" artifacts found

The failure says failed to generate asset "Image": multiple "disk" artifacts found, and when digging around (thanks @prashanths for the help) we've discovered that the error is being thrown from this function (link: https://github.com/openshift/installer/blame/master/pkg/rhcos/builds.go#L80) now that the rcos.json file includes the secure execution qemu image (link: https://github.com/openshift/installer/blob/master/data/data/coreos/rhcos.json#L411).  Now that there are two images, we need to choose the correct image for the installer.