Disable setup-ocaml's redundant internal cache

[cknitt]

Summary

• Upgrade ocaml/setup-ocaml from 3.7.2 to 3.8.0
• Disable its action-managed OPAM cache
• Update our cache keys to include the new setup-ocaml version

Why

We already cache the complete OPAM environment, including project dependencies. setup-ocaml's internal cache covers overlapping paths and creates an additional, less-complete cache before dependencies are installed.

Disabling it avoids redundant cache uploads and reduces pressure on GitHub Actions' cache storage limit. Our existing cache continues to provide warm OPAM environments.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-30T09:21:59.745771Z	f5de8d5	PR opened

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commit: f5de8d5