stable_abi.py --all fails on Windows

The `--all` command [from the devguide](https://devguide.python.org/developer-workflow/c-api/#adding-a-new-definition-to-the-limited-api) fails on Windows.

`--all` should only enable `unixy_check` on Unixy platforms.



### Linked PRs
* gh-105092
* gh-105133