From what I vaguely understand, part of the GitHub terms of service is granting GitHub / Microsoft various rights to store, index, etc. the contents of your repositories. That is, in addition to whatever license you choose to use for your project, you are granting GH/MS additional rights on top of that (if they are not already granted by the project license).

While I am not a lawyer / licensing expert, I would be very surprised if the terms of service did not include rights or some other loophole allowing it to be used for training machine learning systems[1]. So the actual license you'd use is probably irrelevant while it's hosted on GitHub.

[1] A paragraph from section D4 here https://docs.github.com/en/site-policy/github-terms/github-t... states:

"We need the legal right to do things like host Your Content, publish it, and share it. You grant us and our legal successors the right to store, archive, parse, and display Your Content, and make incidental copies, as necessary to provide the Service, including improving the Service over time. This license includes the right to do things like copy it to our database and make backups; show it to you and other users; parse it into a search index or otherwise analyze it on our servers; share it with other users; and perform it, in case Your Content is something like music or video."

I'd guess CoPilot would be covered by either the parsing or improving the service provisions.