Fix Rye not using user-chosen toolchain as default during installation

[pjdon]

Fixes https://github.com/astral-sh/rye/issues/1024

Currently:

• When installation calls ensure_self_venv_with_toolchain to bootstrap rye internals it would pass the toolchain_version_request variable as the version to install
• If the RYE_TOOLCHAIN_VERSION env var is not set, then the value of toolchain_version_request remains None
• During installation, prompt_for_default_toolchain is called to prompt the user for a version to select, asking the user "Which version of Python should be used as default toolchain?"
• This function stores the user's selection in the config_doc TOML configuration and returns nothing
• The user's toolchain version selection is thus never passed to the internals bootstrapping call

Changes:

• This fix makes toolchain_version_request mutable so it can be updated by the user's input
• Function prompt_for_default_toolchain returns the resolved version that the user wants, which may be the input they typed or the default value
• The result is stored in toolchain_version_request so it can be used during the internals bootstrapping

[charliermarsh]

Thanks, this makes sense. I just tweaked the code a bit to respect the RYE_TOOLCHAIN_VERSION if it is set. I could see an argument either way... but I think the set env var overriding a prompted "default" version is reasonable.