nixos/virtualisation: use mkDefault in networking.useNetworkd in oci-common

To make it easier to disable for those folks who don't want to use
systemd-networkd.

nixos/modules/virtualisation/oci-common.nix

···
 
   # Otherwise the instance may not have a working network-online.target,
   # making the fetch-ssh-keys.service fail
-  networking.useNetworkd = true;
+  networking.useNetworkd = lib.mkDefault true;
 }