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 95.45%. Comparing base (a84d179) to head (ff72643).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          16       16           
  Lines        1013     1013           
  Branches       78       78           
=======================================
  Hits          967      967           
  Misses         45       45           
  Partials        1        1           

Flag	Coverage Δ
unittests	95.45% <ø> (ø)

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