Fixes the build for `python3Packages.trio' for the next ZHF iteration.
Please refer to the Hydra build for further reference: https://hydra.nixos.org/build/80617356
`python3Packages.sniffio` is needed for the build, otherwise the build
aborts with an error like this:
```
Could not find a version that satisfies the requirement sniffio (from trio==0.6.0) (from versions: )
No matching distribution found for sniffio (from trio==0.6.0)
```
See #45960
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
Those were removed because they were added to openblas itself. There was
an error with that however and it will take some time until the fix
makes its way through staging. For the time being, reverting this part
of 26d5d030fe will fix the sage build.
This commit should be reverted once
408eacbc89 is in master.
