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=

nixos/modules/system/boot/networkd.nix

···
           "ip6gretap"
           "ipip"
           "ipvlan"
+          "ipvtap"
           "macvlan"
           "macvtap"
           "sit"
···
           "VRF"
           "VLAN"
           "IPVLAN"
+          "IPVTAP"
           "MACVLAN"
           "MACVTAP"
           "VXLAN"