networkd: Fix disabled networkd units.

In f8dbe5f, the default value for networking unit "enabled" option
suddenly flipped to false. I have no idea of whether this happened by
accident, but I'm setting it to true again, because it essentially
breaks systemd networking support and we have systemd.network.enable to
have a "turn the world off" switch.

And of course, because the mentioned commit obviously wasn't done with
even a run of the simplest run of one of the network VM tests, we now
get an evaluation error if we switch useNetworkd to true.

Fixes the core issue of #7505.

Signed-off-by: aszlig <[e-mail]>

nixos/modules/system/boot/networkd.nix

···
   commonNetworkOptions = {
 
     enable = mkOption {
-      default = false;
+      default = true;
       type = types.bool;
       description = ''
         Whether to manage network configuration using <command>systemd-network</command>.