Take two of #40708 (4fe2898608).
That PR attempted to bidirectionally default `config.nixpkgs.system` and
`config.nixpkgs.localSystem.system` to each be updated by the other. But
this is not possible with the way the module system works. Divergence in
certain cases in inevitable.
This PR is more conservative and just has `system` default `localSystem`
and `localSystem` make the final call as-is. This solves a number of
issues.
- `localSystem` completely overrides `system`, just like with nixpkgs
proper. There is no need to specify `localSystem.system` to clobber the
old system.
- `config.nixpkgs.localSystem` is exactly what is passed to nixpkgs. No
spooky steps.
- `config.nixpkgs.localSystem` is elaborated just as nixpkgs would so
that all attributes are available, not just the ones the user
specified.
The remaining issue is just that `config.nixpkgs.system` doesn't update
based on `config.nixpkgs.localSystem.system`. It should never be
referred to lest it is a bogus stale value because
`config.nixpkgs.localSystem` overwrites it.
Fixes#46320
9f9723b179