A single HTTP 502 from the GitHub release CDN killed the entire v0.1.5 tag build (one job of five, ~50 minutes wasted). The build-script download now retries twice with backoff on 5xx/429/network errors; genuine failures (404, offline) still fail fast with the KIGI_SHELL_BUNDLE_RG_PATH hint, and every retry prints a cargo:warning so flakiness stays observable.