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

{
  "name": "test",
  "devDependencies": {
    "mocha": "^9.0.2",
    "ts-node": "^10.4.0"
  },
  "scripts": {
    "test": "mocha --loader node_modules/ts-node/dist/esm.js",
    "test-types": "tsd"
  },
  "mocha": {
    "timeout": 30000
  },
  "dependencies": {
    "tsd": "^0.20.0"
  }
}

{
  "name": "test",
  "devDependencies": {
    "mocha": "^9.0.2",
    "ts-node": "^10.4.0"
  },
  "scripts": {
    "test": "mocha --loader node_modules/ts-node/dist/esm.js",
    "test-types": "tsd"
  },
  "mocha": {
    "timeout": 30000
  },
  "dependencies": {
    "tsd": "^0.20.0"
  }
}
