Explain how to use environment variables in a test matrix

[martin389]

This topic was suggested in https://twitter.com/Benoit_Tgt/status/1324684045712793601 🙇‍♂️ 🎉

What article on docs.github.com is affected?

The "Workflow syntax for GitHub Actions" documentation needs to explain how to add custom environment variables to a test matrix.

What part(s) of the article would you like to see updated?

The proposed example could be a part of the jobs.<job_id>.strategy.matrix docs.

Additional information

Once this has been added to the main docs, we can look at adding something similar to the Travis migration guide.

[welcome]

Thanks for opening this issue. A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.