use of rancher-roles-management units broken in pure-OCI deployments

Some Helm-based units are defined only in in management.values.yaml and/or workload-cluster.values.yaml and not in values.yaml.

For these units:

• tools/oci/push-helm-charts-artifacts.py builds no artifact
• tools/oci/build-sylva-units-artifact.py does not create any overlay to make them point to the OCI artifact for their Helm chart.

This prevents their use in OCI-only deployments: since they are not present in the use-oci-artifacts.values.yaml layer, the GitRepository would be used, which will fail in an OCI-only deployment.

One example is the rancher-roles-management/rancher-cluster-role-bindings units.

/cc @sakuchoudhary98