Add url parameter to install_xbuildenv. ### Description

Resolve #3567 

Adds url paramter to `install_xbuildenv`, which helps installing xbuildenv from arbitrary URL. This is mostly for developers / maintainers who want to test tot xbuildenv.

This PR also adds a private CLI entrypoint `pyodide xbuildenv install`.

### Checklists

- [x] Add / update tests

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

build-backend = "setuptools.build_meta"

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

build-backend = "setuptools.build_meta"
