ci: key opam cache on resolved pins, skip setup-ocaml on warm switch

[tiensonqin]

Summary

Fixes the opam switch cache never actually helping: Install OCaml dependencies took ~2m on every run even with a cache hit.

Root cause: the ~/.opam cache key only covered hashFiles('*.opam'), but all pin-depends float on #main. Once a cache entry exists it's immutable and the save step is skipped on hits, so after any pinned repo (melange-edn, melange-transit, persistent-sorted-set-ocaml, lg) moved, every run restored a stale switch, opam install rebuilt the moved pins, and the rebuild was never re-cached — permanently.

Changes:

• New Resolve pinned dependency commits step git ls-remotes every git+...#ref in *.opam and folds the resulting sha into the opam cache key (...-opam-${hashFiles('*.opam')}-${pins-sha}). A pin bump now produces a new key: partial restore via restore-keys → opam install rebuilds only the moved pins → saved under the new key.
• Any restore (exact or fallback) now skips ocaml/setup-ocaml entirely and just downloads the opam binary (same pattern as logseq/chat CI) — removes the unconditional opam update that cost ~1m40s in Set up OCaml on hits.
• Install OCaml dependencies retries with opam update on failure so a stale repo snapshot in a restored root can't permanently break resolution of newly released versions.
• dune-cache: true moved off setup-ocaml (which no longer runs on hits) to an explicit ~/.cache/dune restore/save with branch-scoped keys plus ~/.config/dune (cache enabled), as in logseq/lg and logseq/chat.

Link to Devin session: https://app.devin.ai/sessions/ae36c1d0df314d38abdca045e69e3708
Open in Devin Desktop: https://app.devin.ai/desktop/session/ae36c1d0df314d38abdca045e69e3708?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

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