nixos/networkd: add ipvtap kind to netdev and IPVTAP= to network
From `man systemd.netdev`: https://www.freedesktop.org/software/systemd/man/latest/systemd.netdev.html#Supported%20netdev%20kinds
From `man systemd.network`: https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#IPoIB=