Fix cache restoration logic to use Promise.allSettled for better error handling

[smorimoto]

Closes #951

Pull Request Overview

This PR improves cache restoration by switching from Promise.all to Promise.allSettled for better error handling in both opam and cygwin caches.

• Replaces Promise.all with Promise.allSettled to prevent failures in one cache from affecting the other.
• Unpacks settled promise results into opam and cygwin cache hit values.

Comments suppressed due to low confidence (1)

packages/setup-ocaml/src/cache.ts:172

• [nitpick] Consider renaming the variables to opamCacheResult and cygwinCacheResult to clearly indicate that these values are the outcomes of Promise.allSettled, which are then subsequently unpacked into cache hit values.

const [opamCache, cygwinCache] = await Promise.allSettled(