nixos/spiped: use systemctl restart during activation

As is common with other networking services, stopIfChanged=true (the default) can cause O(seconds) downtime during activation.

Reduce this downtime by disabling stopIfChanged as done in:
* sshd https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/ssh/sshd.nix#L569
* tailscale https://github.com/NixOS/nixpkgs/pull/170210

nixos/modules/services/networking/spiped.nix

@@ -186,6 +186,7 @@ in
         Restart   = "always";
         User      = "spiped";
       };
+      stopIfChanged = false;
 
       scriptArgs = "%i";
       script = "exec ${pkgs.spiped}/bin/spiped -F `cat /etc/spiped/$1.spec`";