Regression introduced by 943592f698.
The lib attribute isn't in scope here, so we need to use pkgs.lib
instead for isFunction.
Signed-off-by: aszlig <aszlig@nix.build>
Cc: @shlevy
Among other things, this will allow *2nix tools to output plain data
while still being composable with the traditional
callPackage/.override interfaces.
