Tidy Python virtual environment path possibly incorrect on Windows

[Walnut356]

Found when running ./x test tidy --extra-checks py:fmt

Currently the relative path is set to Scripts/python3.exe. I'm not sure if it's for CI runner or legacy reasons or just the usual "nobody really develops on windows", but both Python's built-in venv and uv use python.exe as the executable name.

[Kobzol]

Sounds like an easy fix! If changing to python.exe works for you on Windows, can you send a PR and r? me? Thanks!

[rustbot]

Error: r? is only allowed on PRs.

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