Explicit use bash for Windows

[Olegt0rr]

Description

Windows runners use PowerShell by default, so provided docs are not relevant for this os.

Motivation and Context

Fixes #1220

How Has This Been Tested?

Tested here: https://github.com/aio-libs/aiohttp/pull/8339

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (add or update README or docs)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.