vpn-slice: fix propagated dependencies on darwin

vpn-slice has different requirements for externally available
commands depending on the os:

> Supported OSes:
>   - Linux kernel 3.x+ with iproute2 and iptables utilities (used for all routing setup)
>   - macOS 10.x with BSD route

pkgs/tools/networking/vpn-slice/default.nix

@@ -1,10 +1,12 @@
 { lib
+, stdenv
 , buildPythonApplication
 , nix-update-script
 , python3Packages
 , fetchFromGitHub
 , iproute2
 , iptables
+, unixtools
 }:
 
 buildPythonApplication rec {
@@ -18,10 +20,13 @@ buildPythonApplication rec {
     sha256 = "sha256-T6VULLNRLWO4OcAsuTmhty6H4EhinyxQSg0dfv2DUJs=";
   };
 
-  propagatedBuildInputs = with python3Packages; [ setproctitle dnspython ] ++ [
-    iproute2
-    iptables
-  ];
+  propagatedBuildInputs = with python3Packages; [ setproctitle dnspython ]
+    ++ lib.optionals stdenv.isLinux [
+      iproute2
+      iptables
+    ] ++ lib.optionals stdenv.isDarwin [
+      unixtools.route
+    ];
 
   doCheck = false;