ci(quint): run spec verification without recompiling Rust MBT

[davidabram]

Pull request stack

Review and merge bottom-up. Each PR targets the previous PR's branch, so Files changed shows only that step's incremental changes.

Step	Pull request	Change
1/4	#308 — Faster Nix checks	Rust test-profile cache; deduplicate default MBT check
2/4	#309 — Quint workflow cleanup	Stop recompiling Rust MBT separately
3/4	#310 — macOS smoke-build reuse	Reuse release artifact for macOS smoke tests
4/4	#311 — CI timing summaries	Record Nix phase durations in job summaries

You are reviewing step 2/4: #309. Parent: #308. Next: #310.

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The full Rust cli-tests check already executes the Quint Connect model-based tests on Linux and macOS. Drop the additional Rust MBT build from .github/workflows/quint.yml while retaining Quint typechecking, named spec tests, randomized safety checks, and the required gate job.

The focused nix build .#mutation-trace-quint-connect remains available on demand. Remove the unused Magic Nix Cache action from this now-spec-only workflow, reducing concurrent cache pressure.

Validation: Nix CI must continue to run the entire Rust suite, and Quint gate must pass. Actual CI timings pending.