nixos/tinc: remove ordering dependency on network.target

This allows configuring IP addresses on a tinc interface using
networking.interfaces."tinc.${n}".ipv[46].addresses.

Previously, this would fail with timeouts, because of the dependency
chain
tinc.${netname}.service
--after--> network.target
--after--> network-addresses-tinc.${n}.service (and network-link-…)
--after--> sys-subsystem-net-devices-tinc.${n}.device

But the network interface doesn't exist until tinc creates it! So
systemd waits in vain for the interface to appear, and by then the
network-addresses-* and network-link-* units have failed. This leads
to the network link not being brought up and the network addresses not
being assigned, which in turn stops tinc from actually working.