CI: enhance nightly backward compatibility testing workflow

[dlachaume]

Content

This PR includes enhancements for the nightly backward compatibility testing workflow. It schedules the workflow to run at night (2:30 UTC) and uploads artifacts upon failure.

Additionally, it introduces a comprehensive published summary for the runs:

Pre-submit checklist

• Branch
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

Issue(s)

Closes #2027

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   10m 59s ⏱️ +26s 1 418 tests ±0  1 418 ✅ ±0  0 💤 ±0  0 ❌ ±0  1 629 runs  ±0  1 629 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 1ad5f1db. ± Comparison against base commit fcca57d7.

:recycle: This comment has been updated with latest results.