Copilot never shows as assignee

[alexr00]

To repro:

1. Open a folder that contains a GitHub repo
2. Look at your Issues tree view. Open one of the issues on github.com
3. Assign Copilot to the issue
4. Go back to VS Code.
5. Open the issue in the Issue webview
6. 🐛 Copilot won't show as one of the assignees.

GitHub changed the API such that Bot assignees aren't returned. We need to use a different property to get the bots.