add `codecov_token` secret to CI workflow

[zacharyburnett]

turns out, we have a token but just weren't using it in the CI

Checklist

• [ ] added entry in CHANGES.rst under the corresponding subsection
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)
• [ ] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[zacharyburnett]

I'll wait for the codecov upload step to finish before merging

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.92%. Comparing base (1aec135) to head (2c643e1). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1265 +/- ## ========================================== - Coverage 78.93% 78.92% -0.02% ========================================== Files 117 117 Lines 8086 8065 -21 ========================================== - Hits 6383 6365 -18 + Misses 1703 1700 -3 ``` | [Flag](https://app.codecov.io/gh/spacetelescope/romancal/pull/1265/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | *Carryforward flag | |---|---|---|---| | [nightly](https://app.codecov.io/gh/spacetelescope/romancal/pull/1265/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | `62.79% <ø> (+0.01%)` | :arrow_up: | Carriedforward from [1aec135](https://app.codecov.io/gh/spacetelescope/romancal/commit/1aec135149bc96e7b8f7ef208dc3daa1f9139a9b?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | *This pull request uses carry forward flags. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) to find out more.

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