Set up /etc/machine-id before starting systemd

Otherwise, systemd will try to populate /etc, which we don't want.

nixos/modules/system/boot/systemd.nix

···
         # groups, in addition to those in the systemd-journal group.
         # Users can always read their own journals.
         ${pkgs.acl}/bin/setfacl -nm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal || true
+
+        if ! [ -e /etc/machine-id ]; then
+          ${systemd}/bin/systemd-machine-id-setup
+        fi
       '';
 
     users.extraUsers.systemd-network.uid = config.ids.uids.systemd-network;