fixed codecov

[chiamp]

Fixed codecov error breaking HEAD: {'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected available in 800 seconds.', code='throttled')}

For context, other people are also having issues with this error.

To resolve this error, I followed the steps outlined here:

• go to https://app.codecov.io/ and sign in to your github account
• go to the 'Repos' tab and find Flax and click the "Configure" button
• copy the CODECOV_TOKEN and add it to Github actions (Settings --> Secrets and variables --> Actions --> Repository secrets --> New repository secret)
• copy the following into the .github/workflows/build.yml file:

- name: Upload coverage reports to Codecov
    uses: codecov/codecov-action@v4.0.1
    with:
      token: ${{ secrets.CODECOV_TOKEN }}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.43%. Comparing base (2c7d7cd) to head (b4b9d12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3895   +/-   ##
=======================================
  Coverage   60.43%   60.43%           
=======================================
  Files         105      105           
  Lines       13263    13263           
=======================================
  Hits         8015     8015           
  Misses       5248     5248           

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