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 67.75%. Comparing base (ad9b6a3) to head (9c08730).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #963      +/-   ##
==========================================
+ Coverage   67.27%   67.75%   +0.47%     
==========================================
  Files         128      128              
  Lines        3221     3228       +7     
  Branches      931      913      -18     
==========================================
+ Hits         2167     2187      +20     
+ Misses       1005      996       -9     
+ Partials       49       45       -4     

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