Add a reusable retry helper that detects GitHub API 403 rate-limit errors
during Nix flake evaluation and retries with exponential backoff.
Apply the retry logic to flake-only CI tests so transient GitHub rate
limits no longer cause random CI failures while preserving fast failure
for real errors.
https://chatgpt.com/share/693d7ec5-ac70-800f-a627-ef705c653ba1