Add setFunctionArgs lib function.

Among other things, this will allow *2nix tools to output plain data
while still being composable with the traditional
callPackage/.override interfaces.

nixos/lib/testing.nix

@@ -85,7 +85,7 @@ rec {
 
       testScript' =
         # Call the test script with the computed nodes.
-        if builtins.isFunction testScript
+        if lib.isFunction testScript
         then testScript { inherit nodes; }
         else testScript;