fix(ci): set GITHUB_TOKEN environment variable for gothub

[gebner]

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list