Codecov upload follow-up

[kandersolar]

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

See #2008 and #2022. This PR adds a missing piece of the puzzle: making the codecov upload token available to the codecov upload steps. This is not necessary for PR uploads, but is needed for coverage to be uploaded correctly after merges to main.

After verifying that this new commit doesn't change/break anything regarding the PR upload, I plan to merge and see if it gets uploads working on main as well.

[kandersolar]

The PR upload failed initially due to codecov's own token (the one it uses for PR uploads) hitting a rate limit. That will still be an unpredictable nuisance issue for us. Rerunning the job after a few minutes fixed it.

Merging, and again checking to see if the upload on main succeeds...