CI: Use codecov token to prevent rate limit

In https://github.com/openSUSE/Mojo-IOLoop-ReadWriteProcess/settings/secrets/actions
a CODECOV_TOKEN, originally from
https://app.codecov.io/gh/openSUSE/Mojo-IOLoop-ReadWriteProcess/config/general,
is defined. By using that token we should be able to work around "Rate
limit reached" as found in https://github.com/openSUSE/Mojo-IOLoop-ReadWriteProcess/actions/runs/14136273645/job/39608649818#step:7:98

Author: okurz

.github/workflows/ci-tests.yaml

@@ -40,6 +40,7 @@ jobs:
         uses: codecov/codecov-action@v4
         if: matrix.perl == 'latest'
         with:
+          token: ${{ secrets.CODECOV_TOKEN }}
           file: cover_db/codecov.json
           fail_ci_if_error: true
           verbose: true