networking.hostId: fix cmd in description

1. Simplify the command by reading directly from /etc/machine-id which
is already a random, lower-case hex string
2. Previously, the command output could be too short because of missing
leading digits. This is now fixed.

nixos/modules/tasks/network-interfaces.nix

@@ -341,7 +341,7 @@ in
         You should try to make this ID unique among your machines. You can
         generate a random 32-bit ID using the following commands:
 
-        <literal>cksum /etc/machine-id | while read c rest; do printf "%x" $c; done</literal>
+        <literal>head -c 8 /etc/machine-id</literal>
 
         (this derives it from the machine-id that systemd generates) or