build: Update codecov and use token

[arbrandes]

Update codecov to the latest version and start using the org-wide token for uploads.

See openedx/wg-frontend#179

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.60%. Comparing base (45246ad) to head (3f0f0c6).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
- Coverage   70.67%   70.60%   -0.07%     
==========================================
  Files          24       24              
  Lines         358      364       +6     
  Branches       90       96       +6     
==========================================
+ Hits          253      257       +4     
- Misses        103      105       +2     
  Partials        2        2              

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