image: Switch to oci-chunked format

Now that we have rpm-ostree v2022.2 in the bootimage, we should
be able to switch to the chunked image format.

This matches coreos/fedora-coreos-config#1698

image.yaml

@@ -12,7 +12,7 @@ squashfs-compression: gzip
 # defaulting to `ip=dhcp,dhcp6 rd.neednet=1` when it doesn't see this key.
 ignition-network-kcmdline: []
 
-ostree-format: "oci"
+ostree-format: "oci-chunked"
 
 vmware-os-type: rhel8_64Guest
 # VMware hardware versions: https://kb.vmware.com/s/article/1003746