nixpkgs module: Fix defaulting of `localSystem` and `system`

My c6f7d436 made the mistake of not
having enough defaults. Now both variables are default as the *explicit*
value of the other, or a fallback. The fallback of `system` is the
default of `localSystem.system`. The fallback of `localSystem` is not
the other default (projected), as that would cause a cycle, but `{
system = builtins.currentTime; }` just as nixpkgs itself does it.