Use token for Codecov

[stefan6419846]

At the moment, we are using tokenless uploads to Codecov. This regularly requires to restart the coverage upload job until it has been successful and the Codecov comment is updated on the PR.

Example error output:

info - 2024-05-30 13:29:27,496 -- The PR is happening in a forked repo. Using tokenless upload.
info - 2024-05-30 13:29:27,804 -- Process Commit creating complete
error - 2024-05-30 13:29:27,804 -- Commit creating failed: {"detail":"Tokenless has reached GitHub rate limit. Please upload using a token: https://docs.codecov.com/docs/adding-the-codecov-token. Expected available in 471 seconds."}

The workflow does not fail in this case, thus this is hard to spot if one is not looking for it.

For this reason, we should probably consider adding the corresponding token, which can only be done by users which have access to the GitHub and Codecov settings.