nixos/tinc: unbreak the service

The user is actually tinc.${network}, as Mic92 points out in
https://github.com/NixOS/nixpkgs/pull/171703#discussion_r867506032

Sorry, I broke this in https://github.com/NixOS/nixpkgs/pull/171703 earlier.

coreutils 9.1 chown does not complain in this case with a valid dotted user.

nixos/modules/services/networking/tinc.nix

@@ -383,9 +383,9 @@ in
         };
         preStart = ''
           mkdir -p /etc/tinc/${network}/hosts
-          chown tinc:${network} /etc/tinc/${network}/hosts
+          chown tinc.${network} /etc/tinc/${network}/hosts
           mkdir -p /etc/tinc/${network}/invitations
-          chown tinc:${network} /etc/tinc/${network}/invitations
+          chown tinc.${network} /etc/tinc/${network}/invitations
 
           # Determine how we should generate our keys
           if type tinc >/dev/null 2>&1; then