Currently, our CI code coverage reporting depends upon the unit tests succeeding. Which means if a unit run fails, no coverage report is uploaded to Coveralls for that suite. When the suite fails due to missing coverage, we have no easy way of seeing what coverage loss generated the error. In particular, non-Windows people cannot see the coverage report in https://github.com/fastify/fastify/runs/5259373178?check_suite_focus=true

This pull request adds a new step to our CI that generates coverage reports subsequent to linting and prior to unit testing. The result of this coverage report will be attached to the Actions panel for the CI run as a zip file. This zip file can be downloaded and explored with a web browser.

I think this step could also be used to generate an lcov file to upload to Coveralls and thus skip the final Coveralls step. But I'm not sure how to accomplish that with the various OS and Node.js matrices we use along with Coveralls's "parallel" feature. Probably not worth the effort to figure it out, so I haven't.

The coverage reports generated by this new CI flow target the current Node.js LTS only. This is done to simply the setup, and reduce the CI run time.

This PR also tweaks a couple items in the ci.yml to speed up multiple runs within a single PR lifecycle, and to use the LTS version of Node in the jobs where we were using a single Node version.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct