Add codecov token so upload is more stable

[citizen-stig]

We observing this problem with code coverage:

[2024-04-25T14:44:19.306Z] ['info'] Processing /home/runner/work/nmt-rs/nmt-rs/lcov.info...
[2024-04-25T14:44:19.310Z] ['info'] Detected GitHub Actions as the CI provider.
[2024-04-25T14:44:19.672Z] ['info'] Pinging Codecov: https://codecov.io/upload/v4?package=github-action-3.1.6-uploader-0.7.3&token=*******&branch=master&build=8834657417&build_url=https%3A%2F%2Fgithub.com%2FSovereign-Labs%2Fnmt-rs%2Factions%2Fruns%2F8834657417%2Fjob%2F24256874935&commit=ac03d7c7620bab1e14256aa2fde8e425631a5278&job=Rust&pr=&service=github-actions&slug=Sovereign-Labs%2Fnmt-rs&name=&tag=&flags=&parent=
[2024-04-25T14:44:19.855Z] ['error'] There was an error running the uploader: Error uploading to [https://codecov.io:](https://codecov.io/) Error: There was an error fetching the storage URL during POST: 404 - {'detail': ErrorDetail(string='Unable to locate build via Github Actions API. Please upload with the Codecov repository upload token to resolve issue.', code='not_found')}
Error: Codecov: Failed to properly upload: The process '/home/runner/work/_actions/codecov/codecov-action/v3/dist/codecov' failed with exit code 255

It suggest that adding CODECOV_TOKEN might resolve it:

steps:
- uses: codecov/codecov-action@v3
  with:
    token: ${{ secrets.CODECOV_TOKEN }}