Somehow I thought it worked automatically, but we had to provide the
token.
The configuration is based on Coveralls instructions to run parallel
builds [1].
Alternatively we could use the Coveralls GitHub Action [2] which
slightly simplifies the workflow configuration and removes the
dependency of the coveralls gem. However, it also adds a dependency on
simplecov-lcov and requires configuring it to renerate LCOV files on
each run, so the benefits of using it are not that big.
[1] https://docs.coveralls.io/parallel-build-webhook
[2] https://github.com/coverallsapp/github-action/
45693e084b