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

[build-system]
requires = ["setuptools>=42", "wheel"]
build-backend = "setuptools.build_meta"

[project]
name = "pkg_test_executable"
version = "1.0.0"
authors = []

[build-system]
requires = ["setuptools>=42", "wheel"]
build-backend = "setuptools.build_meta"

[project]
name = "pkg_test_executable"
version = "1.0.0"
authors = []
