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

package:
  name: libtest
  version: 1.0.0
source:
  sha256: deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef
  url: https://dummy-url-that-not-exists.com/libtest.tar.gz
build:
  type: static_library
  script: "pass"

package:
  name: libtest
  version: 1.0.0
source:
  sha256: deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef
  url: https://dummy-url-that-not-exists.com/libtest.tar.gz
build:
  type: static_library
  script: "pass"
