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

0000000000000000000000000000000000000000 f1498a2747d5dc968dad2d872181d9cad2d80a48 jupyter <jupyter@data-collection2-susnato.c.rishabh-experiments.internal> 1705136699 +0000	clone: from https://github.com/pyodide/pyodide.git

0000000000000000000000000000000000000000 f1498a2747d5dc968dad2d872181d9cad2d80a48 jupyter <jupyter@data-collection2-susnato.c.rishabh-experiments.internal> 1705136699 +0000	clone: from https://github.com/pyodide/pyodide.git
