Fix mkApp

Using lib.getExe makes mkApp not work for derivations that are files.
Thus, this makes it so that the derivation is used as is. If an
executable in the derivation's directory is needed, a string path can be
used.

default.nix

@@ -153,13 +153,7 @@ let
 
       mkApp = lib: app:
         if isApp app then app
-        else {
+        else { type = "app"; program = "${app}"; };
-          type = "app";
-          program =
-            if lib.isDerivation app
-            then lib.getExe app
-            else app;
-        };
 
       eachSystem = fn: foldAttrs mergeAttrs { } (map
         (system: mapAttrs