Add RTX_PYENV_REPO env

[dfang]

rtx-python use pyenv to download and build python, when you run rtx ls-remote python or commands like that. you need pyenv installed in plugins/python directory.

in China, without vpn, the connection to GitHub is unstable, with this RTX_PYENV_REPO env, we can improve rtx experience drastically.

eg.

# for pyenv in rtx-python plugin
export RTX_PYENV_REPO="https://ghproxy.com/https://github.com/pyenv/pyenv.git"

# for rtx install python@xxxx (from pyenv)
export PYTHON_BUILD_MIRROR_URL_SKIP_CHECKSUM=false
export PYTHON_BUILD_MIRROR_URL=https://repo.huaweicloud.com/python/

[dfang]

jdx/mise#677