Support private repositories for pipx/uvx

[clement-tourriere]

For the moment Mise only supports package installation from PYPI (and github/git url).

Could you add the possibility to support package installation from custom python package index url via custom --index-url option for pipx and --default-index parameter for uv(x) or even by defining a UV_DEFAULT_INDEX variable inmise.config.toml

[jdx]

I think there's something like pipx_install_args and install_env (check docs) which can be used to set either of these