Cover the use of WSL in CI

Background

Some users are unable to use a pure linux environment, and the next best thing for a Windows user is WSL.

Thanks to !726 (merged), WSL works, after a fashion, but we are currently lacking coverage of this in gitlab CI.

Task description

• Devise a prototype of running buildstream's CI on WSL
• Devise a reliable way to set up and configure a WSL CI runner
• Submit documentation for setting up and confgiuring a WSL CI runner
• Submit a MR extending .gitlab-ci.yml to include a WSL-specific test

Acceptance Criteria

1. buildstream's CI runs tests on a WSL runner.
2. A user with minimal experience of WSL and gitlab internals can follow instructions to add a new WSL runner.

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