gh-125398: Convert paths in venv activate script when using Git Bash under Windows...

gh-125398: Convert paths in venv activate script when using Git Bash under Windows (GH-125399)

* Convert paths in venv activate script when using Git Bash under Windows

With https://github.com/python/cpython/pull/112508 the check to converts paths when running on Windows was changed from using the non-posix environment variable `$OSTYPE` to using `uname` instead.

However this missed the fact that when running under Git Bash on Windows, uname reports `MINGW*` (`$OSTYPE` is still `msys`).

This results in `$PATH` being set to something like `D:\a\github-actions-shells\github-actions-shells\venv/Scripts:…`, instead of `/d/a/github-actions-shells/github-actions-shells/venv/Scripts`.

Notably, the Git Bash is the bash shell that’s used for GitHub Actions Windows runners, and ships with VSCode.