Out-of-tree recipe build support. ### Description

Add out-of-tree support of `pyodide build-recipes`. Thanks to the awesome xbuildenv work by Hood, the required code change was quite small.

I am testing this branch here: https://github.com/ryanking13/pyodide-recipes-mirror

### Checklists

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

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).
