test-coverage.yaml should fail on error

[Moohan]

debendabot recommended updating the codecov action from 4 to 5 in our test-coverage.yaml. In the release notes, there are some breaking changes that will affect the current r-lib example: file: becomes files: and plugin: becomes plugins: I was surprised to see the workflow had passed on the PR though... Looking at the job details - https://github.com/Public-Health-Scotland/phsopendata/actions/runs/11883091545/job/33109379336 we can see that the action reported a warning and an error but the overall workflow still reported passing for some reason.

I'm not sure if this is something you can update or if it's part of the codecov action itself.