Fix artifact path for simplecov report upload

[johansenja]

What? Why?

Resolves issue in comment here: #12798 (review); only files in the root of the coverage directory were being uploaded, not nested files

What should we test?

• Go to the assets after the CI run
• Download the report
  • The directory should now contain the assets folder
• Open index.html
  • the page should now be properly styled

Release notes

• Technical changes only

The title of the pull request will be included in the release notes.

Dependencies

Documentation updates

[johansenja]

Much better than the old one!

[mkllnk]

Nice! That works well.

I was worried about using space for something that's not important but the zip size is only increased by 120kb (from 862kb to 980kb). I think it's worth it.