nixos/tinc: do not tell systemd where is pidfile

```Tinc```'s pid file has more info than just a pid

```
# cat /run/tinc.dmz.pid
12209 7BD4A657B4A04364D268D188A0F4AA972A05247D802149246BBE1F1E689CABA1 127.0.0.1 port 656
```
so ```systemd``` fails to parse it.
It results in long (re)start times when ```systemd``` waits for a correct pid file to appear.