image: Switch to oci-chunked-v1 format

We now have a version of rpm-ostree in the 4.12 bootimage which
is able to pull this.

This matches coreos/fedora-coreos-config#1698

image-rhel-8.6.yaml

@@ -8,7 +8,9 @@ squashfs-compression: gzip
 # defaulting to `ip=dhcp,dhcp6 rd.neednet=1` when it doesn't see this key.
 ignition-network-kcmdline: []
 
-ostree-format: "oci"
+# See https://github.com/coreos/coreos-assembler/pull/3014 - this
+# will hopefully be the default soon, but trying to enable it here first.
+ostree-format: "oci-chunked-v1"
 
 # vmware-secure-boot changes the EFI secure boot option.
 # set false here due to https://bugzilla.redhat.com/show_bug.cgi?id=2106055