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,
```
