github/workflows: add token for keepalive

[leandrolanzieri]

As discussed in #103, the keepalive workflow step is failing due to permissions to push to master. Instead of removing all protections for master, we now allow admins to push, and added @riot-ci to the group. We then created an access token for @riot-ci, added it to the repo's secrets and now it's being used.

We're not 100% this is the final solution, more settings may need tweaking...

@miri64

[github-actions[bot]]

🚀 PR preview deployed to https://RIOT-OS-riot-os-org-preview-104.surge.sh