Fix flaky cask CI matrix test

- Fix `contains_os_specific_artifacts?` memoization:
  `||=` doesn't cache `false`, so the method re-ran
  the full OS/arch simulation loop on every call,
  risking stale state. Use nil/false/true trichotomy.
- Reset `@contains_os_specific_artifacts` in `refresh`
  so the cache is invalidated when `@dsl` changes.
- Clear `SimulateSystem` in spec_helper's `ensure`
  block to prevent leaked simulation state between
  test examples.