GitHub Actions: add CODECOV_TOKEN to env

[codecov[bot]]

Codecov Report

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

Comparison is base (4053f69) 85.33% compared to head (8f26b53) 85.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3911 +/- ## ======================================= Coverage 85.33% 85.33% ======================================= Files 234 234 Lines 55960 55960 ======================================= Hits 47751 47751 Misses 8209 8209 ```

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