Define CODECOV token

[jack-berg]

See @trask's comment here: #6183 (comment)

As of codecov 4.0.0:

Tokenless uploading is unsupported. However, PRs made from forks to the upstream public repos will support tokenless (e.g. contributors to OS projects do not need the upstream repo's Codecov token). This doc shows instructions on how to add the Codecov token.

My read of this is that builds for this repo will need the token to upload, but PRs from forks don't need it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e9266c8) 91.06% compared to head (ffb41ad) 91.06%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6186   +/-   ##
=========================================
  Coverage     91.06%   91.06%           
  Complexity     5672     5672           
=========================================
  Files           620      620           
  Lines         16560    16560           
  Branches       1690     1690           
=========================================
  Hits          15081    15081           
  Misses          993      993           
  Partials        486      486           

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