My c6f7d4367894047592cc412740f0c1f5b2ca2b59 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.
4fe2898608