keynav: use correct ExecStart command

When the change requested in

  https://github.com/rycee/home-manager/pull/1082#discussion_r392715440

was applied, the service `ExecStart` attribute was not updated to use
`pkgs.keynav`.

Fixes #1177
PR #1184

(cherry picked from commit ee1c40e5c5)

modules/services/keynav.nix

@@ -18,7 +18,7 @@ in {
       };
 
       Service = {
-        ExecStart = "${cfg.package}/bin/keynav";
+        ExecStart = "${pkgs.keynav}/bin/keynav";
         RestartSec = 3;
         Restart = "always";
       };