Enhancement: python.uv_venv_auto should take into account UV_PROJECT_ENVIRONMENT

[rsyring]

uv's default is to use .venv for a project's virtualenv. However, a different path can be provided by using UV_PROJECT_ENVIRONMENT.

Currently, mise doesn't check for an alternate location.

This is probably obvious, but the variable being used should come from the result of mise's environment variable processing pipeline.

My current workaround is to set:

[env]
_.python.venv.path = '{{ env.UV_PROJECT_ENVIRONMENT }}'
_.python.venv.create = true

But it would be cleaner if I could just:

[settings]
python.uv_venv_auto = true

and trust that whatever uv is doing, mise will play along.

FWIW, my higher-level goal is to support some of our users wanting to use .venv and some users wanting to put those files in a temporary/cache location. So, our actual project files look like:

[env]
VENV_IDENT = 'foo-app'

_.python.venv.path = '{{ env.VIRTUALENV_DIR }}'
_.python.venv.create = true

And then ~/mise.toml has:

[env]
# VIRTUALENV_DIR = '.venv'

or:

[env]
VIRTUALENV_DIR = { value='{% if env.VENV_IDENT %}~/.cache/venvs/{{ env.VENV_IDENT }}{% endif %}', tools = true }
UV_PROJECT_ENVIRONMENT = { value='{% if env.VIRTUALENV_DIR %}{{ env.VIRTUALENV_DIR }}{% endif %}', tools = true }