nixos/networkd: continue supporting 99-main with wildcard interface match

With systemd version 243 network units with empty match block will
generate warnigs. The reasoning seems to be that the intended behaviour
is hard to infere. Being explicit about really meaning any interface is
the reasonable thing here.

We want to get rid of this mechanism in the long run but as long as we
do not have a replacement we should stick with it and keep it in
reasonable good shape.