Coveralls: Pass $GITHUB_TOKEN to get coveralls.io working again

[cclauss]

Fix these errors...

• https://github.com/Kozea/pygal/actions/runs/7168325592/job/19516257331 under Run coveralls...

  Run coveralls
  Running on Github Actions but GITHUB_TOKEN is not set. Add "env: GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}" to your step config.
  Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/coveralls/cli.py", line 64, in main
  coverallz = Coveralls(token_required,
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/coveralls/api.py", line 49, in __init__
  self.ensure_token()
  File "/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/coveralls/api.py", line 56, in ensure_token
  raise CoverallsException(
  coveralls.exception.CoverallsException: Running on Github Actions but GITHUB_TOKEN is not set. Add "env: GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}" to your step config.

• https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-support

  --> https://coveralls.io/github/Kozea/pygal

  Scroll down to see that these are the first coveralls updates for this project since 2017.