CI: cache and retry release-asset downloads (Weaver, envtest, helm-unittest) #577

Problem

giantswarm/substrate's CI downloads release assets straight from github.com, with no cache and no retry: Weaver, setup-envtest's envtest-v1.36.2 and the helm-unittest 1.0.3 checksum. During GitHub's release-download 504s on 2026-11-09, each of these failed reruns of #568 for reasons unrelated to the change, and every Substrate PR on the line was held.

Proposed solution

Fetch these assets with retries and backoff, and cache them by version in CI (CircleCI cache or GitHub Actions cache), or mirror them to gsoci.azurecr.io and pull from there.

Acceptance criteria

• A 504 from github.com release downloads no longer fails a run when the asset is cached, and is retried otherwise.
• The three assets above resolve from the cache on a warm run.

(Filed by an agent, the lab guide, from a worker's report.)