Add CODECOV_TOKEN to the corresponding GH Action

[waldyrious]

The upgrade from the Codecov GitHub Action from v3 to v4 done in #53 required the addition of the CODECOV_TOKEN environment variable to actually work — see #53 (comment).

I first went into the Codecov settings for this repo to obtain the token value, but then I realized that this variable is already available as a secret variable for GitHub Actions in the JuliaMath organization level. So the only thing that needs to be done (IIUC) is adding the line to GitHub Actions workflow .yml file specifying that the codecov-action should run with access to this secret — which is what this PR does.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.23%. Comparing base (e8a0d55) to head (c8e3366).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master      #58      +/-   ##
===========================================
- Coverage   100.00%   95.23%   -4.77%     
===========================================
  Files            1        1              
  Lines           13       21       +8     
===========================================
+ Hits            13       20       +7     
- Misses           0        1       +1     

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[waldyrious]

Looks like the presence of the codecov report comment above is proof enough that this is now working, but I'll wait for @giordano's approval :)