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.

Changed files
+1 -1
nixos
modules
virtualisation
+1 -1
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;
}