Add Codecov token - Githubissues

Description of proposed changes

Not necessary for public repos but decreases chance of upload failures due to GitHub rate limiting.

[x] Set CODECOV_TOKEN as a repository Actions secret (value retrieved from codecov repo settings)
[x] Checks pass
[x] Codecov upload is successful
[x] ~If making user-facing changes, add a message in CHANGES.md summarizing the changes in this PR~ no functional changes