Extract ISO from multi-arch payload

Currently when we extract the RHCOS ISO from a multi-arch release payload, it will fail unless the CPU architecture of the host the installer is running on is the same as the CPU arch of the cluster. So e.g. installing an ARM cluster using an x86 laptop to run the installer, or installing an x86 cluster using an M1 Mac to run the installer, will fail to get the base ISO.

Currently single-arch payloads are the default, but multi-arch payloads are a prerequisite for architecture-heterogeneous clusters.

The machine-os-images container image contains only 1 ISO - the one matching the architecture of the container. This means that in a multi-arch payload there is only one copy of each architecture's ISO, but they are stored in different images (i.e. the x86_64 ISO is stored in the amd64 machine-os-images image; the aarch64 ISO is stored in the arm64 machine-os-images image).

I think that passing a --filter-by-os parameter to oc image extract (to select a particular architecture, rather than defaulting to the local architecture) will be sufficient, but we will have to test.