Fix CI coverage for PR's

[kinow]

Describe the bug

Coverage is now almost working after #324 , but unfortunately looks like the secret token is not available for PR's when the branch is in another repository.

Release version(s) and/or repository branch(es) affected?

Steps to reproduce the bug

Submit a PR from a branch in your fork. Check the GitHub action console output for the step to upload coverage report.

Expected behavior

Screenshots

Additional context

See: codecov/codecov-action#29

Maybe related to codecov/codecov-node#160, if we decided to ditch their github-action and just use the codecov node utility.

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).

[kinow]

The issue in the Codecov GitHub action was fixed & released 4 days ago. My most recent PR to Cylc UI was from 8 days ago, which failed to upload coverage:

So closed, then re-opened that PR, and it just successfully uploaded the coverage report 🎉

Closing!