CI: Expose simplecov's resultset.json for local inspection

What does this MR do and why?

Currently there's no way of download the resulting .resultset.json (created by simplecov). Instead we'd need to download artifacts from all rspec jobs - which is tedious.

This MR adds coverage/.resultset.json as an artifact so it can easily be downloaded and inspected locally (for example: benchmarking simplecov).

Refs #383368

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.