ci: retry Coveralls upload and pin action to commit SHA

[thorstenalpers]

Summary

• Retry the Coveralls upload once after a 60 s pause when the first attempt fails, so transient coveralls.io 5xx errors no longer break CI. A persistent failure still fails the build via the retry step.
• Pin coverallsapp/github-action to the full commit SHA of v2.3.8 (the commit v2 currently points to).

Test plan

• CI passes on this PR (Coveralls upload succeeds on first attempt; retry steps are skipped)

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud