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

# setup: from decimal import Decimal ; A = [Decimal('2.1') for i in range(1000)] ; B = [Decimal('3.2') for i in range(1000)]
# run: large_decimal_list(A, B)


def large_decimal_list(A, B):
    return [a * b for a, b in zip(A, B, strict=True)]

# setup: from decimal import Decimal ; A = [Decimal('2.1') for i in range(1000)] ; B = [Decimal('3.2') for i in range(1000)]
# run: large_decimal_list(A, B)


def large_decimal_list(A, B):
    return [a * b for a, b in zip(A, B, strict=True)]
