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

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["test"], pytest_assert_rewrites=False)
def test_test(selenium):
    # this is tested in test_core_python.py instead
    pass

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["test"], pytest_assert_rewrites=False)
def test_test(selenium):
    # this is tested in test_core_python.py instead
    pass
