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 96.23%. Comparing base (3becef3) to head (34d20ca).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #363   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         184      184           
  Lines        1751     1751           
  Branches      308      308           
=======================================
  Hits         1685     1685           
  Misses         62       62           
  Partials        4        4           

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