Show interpreter source during Python discovery query errors (#7928)
Closes https://github.com/astral-sh/uv/issues/4154
e.g.
```
❯ UV_PYTHON=/dev/null cargo run -q -- pip install anyio
error: Failed to inspect Python interpreter from provided path at `/dev/null`
Caused by: Failed to query Python interpreter at `/dev/null`
Caused by: Permission denied (os error 13)
❯ VIRTUAL_ENV=/dev/null cargo run -q -- pip install anyio
error: Failed to inspect Python interpreter from active virtual environment at `/dev/null/bin/python3`
Caused by: Failed to query Python interpreter
Caused by: failed to canonicalize path `/dev/null/bin/python3`
Caused by: Not a directory (os error 20)
```
