Retry apt installs in ubuntu-dev

[thweetkomputer]

Summary

• add an apt-install-retry helper that clears apt lists, refreshes package indexes, and retries installs
• use it for ubuntu-dev apt installs, including the Python 3.8/deadsnakes layer that failed on arm64 due to a transient Ubuntu ports 404

Validation

• git diff --check
• Docker build not run locally because docker is not installed on this machine; the ubuntu-dev workflow will validate amd64 and arm64 builds

[thweetkomputer]

Closing this follow-up because the arm64 apt 404 appears to have been a transient Ubuntu ports mirror issue; rerunning the existing ubuntu-dev build should be enough.