Add coveralls token to GH action config.

[ntouran]

This secret is obtained from the coveralls page and loaded using the github secrets feature.

We realized we needed this when coveralls started giving an error message that said we needed
to include this env variable.

The error message we saw was:

  Wrote HTML report to htmlcov/index.html
  report: commands[2]> coveralls --service=github
  Not on TravisCI. You have to provide either repo_token in .coveralls.yml or set the COVERALLS_REPO_TOKEN env var.
  Traceback (most recent call last):
    File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/cli.py", line 64, in main
      coverallz = Coveralls(token_required,
    File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/api.py", line 49, in __init__
      self.ensure_token()
    File "/home/runner/work/armi/armi/.tox/report/lib/python3.10/site-packages/coveralls/api.py", line 61, in ensure_token
      raise CoverallsException(
  coveralls.exception.CoverallsException: Not on TravisCI. You have to provide either repo_token in .coveralls.yml or set the COVERALLS_REPO_TOKEN env var.
  report: exit 1 (0.16 seconds) /home/runner/work/armi/armi> coveralls --service=github pid=4199
    cov2: OK (978.37=setup[45.03]+cmd[933.23,0.11] seconds)
    report: FAIL code 1 (13.47=setup[3.72]+cmd[2.25,7.34,0.16] seconds)
    evaluation failed :( (991.99 seconds)
  Error: Process completed with exit code 255.

Fixes #1017

[ntouran]

This got farther, but failed with a new error:

cov2: OK ✔ in 23 minutes 4.73 seconds
report: failed with pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'TOXENV CI GITHUB_*'
  cov2: OK (1384.73=setup[54.34]+cmd[1330.25,0.14] seconds)
  report: FAIL code 1 (0.00 seconds)
  evaluation failed :( (1384.93 seconds)
Error: Process completed with exit code 255.

[john-science]

I added a commit to try and use the Coveralls.io Action. Hopefully that works.

[john-science]

@ntouran After much experimentation, this appears to be working. Though... I had a laughable number of commits to get this working.

Not only did I need to use a Coveralls.io Workflow, I had to add a second coveragepy-lcov workflow, because coveralls.io no longer accepts .coverage.cov file types that Python outputs.