nixos/networkd: Fix restartTriggers

1d61efb7 accidentially changed the
restartTriggers of systemd-networkd.service` to point to the attribute
name (in this case, a location relative to `/etc`), instead of the
location of the network-related unit files in the nix store.

This caused systemd-networkd to not get restarted on activation of new
networking config, if the file name hasn't changed.

Fix this, by pointing this back to the location in the nix store.