Coveralls parallel report fails on GitHub Actions reruns #90

Rerunning the Tests workflow can fail in the Coveralls upload even when lint and all tests pass.

Reproduced in PR #89, workflow run 37370946586, attempt 2. The original attempt suffered hosted-runner assignment failures, and the coverage job (if: always()) finalized its partial report. GitHub reuses github.run_id on a rerun, so the reporter reused the closed build:

Error: Unprocessable Entity
Response: {"message":"Can't add a job to a build that is already closed. Build 37370946586 is closed. See docs.coveralls.io/parallel-builds","error":true}

The workflow should give each attempt a distinct Coveralls build identity and use the same identity for every parallel upload and the finalization job. The supported COVERALLS_SERVICE_NUMBER override can combine github.run_id and github.run_attempt. Preserve all matrix flags and the combined report; validate with a complete workflow rerun.