Right now the rpm-ostree code only enables rhsm (subscription-manager) when operating on `/`. Our "from scratch" flow is impacted by this in the subscription-manager case.

A workaround is to do a dummy "dnf" operation like "dnf repolist" in:

FROM registry.redhat.io/rhel9/rhel-bootc:9.6 as builder
RUN dnf repolist && /usr/libexec/bootc-base-imagectl build-rootfs --manifest=minimal /target-rootfs

But we should change build-rootfs to handle this automatically.