uv

bbd9adaa - Env variables for python downloads (#6416)

Env variables for python downloads (#6416) ## Summary Resolves #6406 ## Test Plan ``` ❯ UV_PYTHON_PREFERENCE=only-managed cargo run -q -- sync --show-settings | rg python_preference python_preference: OnlyManaged, ❯ UV_PYTHON_PREFERENCE=system cargo run -q -- sync --show-settings | rg python_preference python_preference: System, ❯ UV_NO_PYTHON_DOWNLOADS=1 cargo run -q -- sync --show-settings | rg python_downloads python_downloads: Never, ❯ UV_ALLOW_PYTHON_DOWNLOADS=1 cargo run -q -- sync --show-settings | rg python_downloads python_downloads: Automatic, ```