Use loky.cpu_count() instead of os.cpu_count(). <!-- Thank you for contributing to Pyodide! All improvements are welcome,
     so don't be afraid to make a PR. -->

### Description

As discussed in https://github.com/pyodide/pyodide/pull/3559#issuecomment-1435965170

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests

# pyodide-build

Tools for building Pyodide.

See [http://github.com/pyodide/pyodide](http://github.com/pyodide/pyodide) for
more information.

## License

Pyodide uses the [Mozilla Public License Version
2.0](https://choosealicense.com/licenses/mpl-2.0/).

# pyodide-build

Tools for building Pyodide.

See [http://github.com/pyodide/pyodide](http://github.com/pyodide/pyodide) for
more information.

## License

Pyodide uses the [Mozilla Public License Version
2.0](https://choosealicense.com/licenses/mpl-2.0/).
