Support custom python download url or domain

[boringplay]

In some case, we maybe use github proxy to download github contents. Can it support custom python download url or domain ?

For example, "{PYTHON_DOWNLOAD_URL}/{tag}/{filename}" or "{PYTHON_DOWNLOAD_HOST}/astral-sh/python-build-standalone/releases/download/{tag}/{filename}"

mise/src/plugins/core/python.rs

Line 199 in 21aac24

"https://github.com/astral-sh/python-build-standalone/releases/download/{tag}/{filename}"