pasystray: add paprefs and pavucontrol

This enables the "volume control" and "control local sound server"
menu options.

Fixes #461

(cherry picked from commit 6d2f16a7ae)

modules/services/pasystray.nix

@@ -24,6 +24,11 @@ with lib;
         };
 
         Service = {
+          Environment =
+            let
+              toolPaths = makeBinPath [ pkgs.paprefs pkgs.pavucontrol ];
+            in
+              [ "PATH=${toolPaths}" ];
           ExecStart = "${pkgs.pasystray}/bin/pasystray";
         };
     };