nix-netboot-serve skips stage1 initrd

[delroth]

The way nix-netboot-serve currently works boots straight into the stage2 init (the one in the Nix store) and ignores whatever would run in the "normal" initrd for the NixOS configuration being netbooted. This means, for example, that:

• There is no support for pivot_root-ing to a non-tmpfs / filesystem.
• Things like boot.initrd.postMountCommands never get run.
• This probably relies on unstated assumptions of how the stage1 -> stage2 handover works. It's unclear how that will for example work with systemd-stage1.