ci: fix missing CODECOV_TOKEN for codecov/codecov-action@v4

[aofei]

For goplus/gop#1822

---

‼️ Steps that must be done before merging:

1. Obtain the upload token (CODECOV_TOKEN) from https://codecov.io/github/goplus/mod/settings
2. Add CODECOV_TOKEN as a repository secret at https://github.com/goplus/mod/settings/secrets/actions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.35%. Comparing base (9cff459) to head (a37eb82).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   90.63%   90.35%   -0.28%     
==========================================
  Files           6        6              
  Lines         726      726              
==========================================
- Hits          658      656       -2     
- Misses         49       50       +1     
- Partials       19       20       +1     

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