nixos-rebuild switch hangs a host whose root is on iSCSI

One server in jupiter-os has no local disk for its root. It boots over the network and mounts its root filesystem from an iSCSI volume on the NAS.

nixos-rebuild switch activates a new configuration in place, restarting any service whose definition changed. When that service is iscsid, the switch stops the daemon that holds the root disk, and the host hangs hard with no way to finish the activation.

The host is deployed with nixos-rebuild boot, which installs the new generation and applies it at the next restart.