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.

volth b32b1863 7e5332c8

Changed files
-1
nixos
modules
services
networking
-1
nixos/modules/services/networking/tinc.nix
···
path = [ data.package ];
serviceConfig = {
Type = "simple";
-
PIDFile = "/run/tinc.${network}.pid";
Restart = "always";
RestartSec = "3";
};