Use environment secrets for Codecov token

[djc]

It seems like there are some unexpected differences between "Repository secrets" and "Environment secrets" for GitHub repositories. In another repo, this seemed to help. (I've copied the token into the new Environment.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.75%. Comparing base (51c2d4b) to head (9ff8ba0).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1596      +/-   ##
==========================================
- Coverage   91.78%   91.75%   -0.04%     
==========================================
  Files          37       37              
  Lines       18207    18207              
==========================================
- Hits        16712    16706       -6     
- Misses       1495     1501       +6     

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[pitdicker]

We can't easily test this I think? What do you think about adding it as a commit to #1594?

[djc]

It worked in this PR, right? Can rebase #1594 on top of this after it merges -- it certainly doesn't make anything worse.