Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

I don't think Coveralls is checking things correctly. When we look at the CI results for commit 5a77a6b we can see that Coveralls is suggesting coverage remains at 100% for all OSes, but tap is saying otherwise:

https://coveralls.io/jobs/94867280

https://github.com/fastify/fastify/runs/5267302531?check_suite_focus=true

With #3717 and #3719, we will have coverage reports attached to the GitHub Actions panel itself. We'd have to download the artifacts to inspect what coverage is missing, instead of browsing a hosted website (Coveralls), but at least it'd be consistent and accurate(?).