nspawn: provide $container and $container_uuid in /run/host too

nspawn: provide $container and $container_uuid in /run/host too

This has the major benefit that the entire payload of the container can
access these files there. Previously, we'd set them only as env vars,
but that meant only PID 1 could read them directly or other privileged
payload code with access to /run/1/environ.