ci: switch coveralls to repo token auth

[BeiBaiNian]

Summary

Fix the coveralls 422 error that makes all test jobs of the build workflow fail. After the repo moved from casbin/pycasbin to apache/casbin-pycasbin, the coveralls GitHub Actions integration (service_name=github) can no longer match CI jobs to the repository, so every coverage upload fails with 422 Couldn't find a repository matching this job (tests themselves all pass).

Switch to the officially recommended repo-token method:

• Upload step now runs coveralls --service-name=github-actions with COVERALLS_REPO_TOKEN
• The coveralls --finish step uses the same token, replacing GITHUB_TOKEN

Changes

• .github/workflows/build.yml
  • Upload coverage step: run: coveralls --service-name=github-actions; env GITHUB_TOKEN replaced with COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
  • Finish step (coveralls --finish): env GITHUB_TOKEN replaced with COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}

Verification

• Repo-token method verified against coveralls.io before submitting: a POST to https://coveralls.io/api/v1/jobs with the repo token returned 200 {"message":"Job ##1.1","url":"https://coveralls.io/jobs/185704267"} — coveralls accepted the upload.
• YAML syntax and workflow structure validated locally.

Note for maintainers

This PR requires the repository secret COVERALLS_REPO_TOKEN , otherwise CI cannot pass.