Respect `.python-version` files in `uv run` outside projects (#6361)
Closes https://github.com/astral-sh/uv/issues/6285
Introduces a new problem if the user says `python` but it doesn't exist
with that name:
```
❯ cargo run -q -- -v run python --version
DEBUG uv 0.3.0
DEBUG Found project root: `/Users/zb/workspace/uv`
DEBUG Project `uv` is marked as unmanaged
DEBUG No project found; searching for Python interpreter
DEBUG Reading requests from `/Users/zb/workspace/uv/.python-version`
DEBUG Searching for Python 3.11 in managed installations or system path
DEBUG Found `cpython-3.12.4-macos-aarch64-none` at `/Users/zb/workspace/uv/.venv/bin/python3` (virtual environment)
DEBUG Searching for managed installations at `/Users/zb/Library/Application Support/uv/python`
DEBUG Found `cpython-3.11.9-macos-aarch64-none` at `/opt/homebrew/bin/python3.11` (search path)
DEBUG Using Python 3.11.9 interpreter at: /opt/homebrew/opt/python@3.11/bin/python3.11
DEBUG Running `python --version`
error: Failed to spawn: `python`
Caused by: No such file or directory (os error 2)
```
I'll fix this separately.