Switch to using the coveralls GitHub action

[aalykiot]

Since using the coveralls npm module breaks the CI on PRs coming from forks:

/home/runner/work/npcheck/npcheck/node_modules/coveralls/bin/coveralls.js:19
      throw err;
      ^
Bad response: 422 {"message":"Couldn't find a repository matching this job.","error":true}
(Use `node --trace-uncaught ...` to show where the exception was thrown)
Error: Process completed with exit code 1.

It would be great if we move the coveralls checks on the CI layer.

@richardlau @helio-frota if any of you have time to take this one would be great since my CI knowledge is limited.