[CI] Mac jar build uses a different LLVM version than Linux

### Describe the bug, including details regarding any error messages, version, and platform.

The Mac build uses an older LLVM version (15) than Linux (18). This seems to be related to the CI code and the fact that the mac github runners have llvm installed via brew
https://github.com/actions/runner-images/blob/releases/macos-14-arm64/20250915/images/macos/macos-14-arm64-Readme.md
Even newer runners only have LLVM 17.

The discrepancy could lead to hard to find bugs or performance issues.

Is it possible to use vcpkg for the mac builds? That way we could be assured that they are always consistent across platforms.

A few related changes I found:
Unpin the llvm version in arrow/cpp/Brewfile. I think this might mean that theoretically the latest llvm could be installed but since the runner image already has an older one installed, that is used instead.
https://github.com/apache/arrow/commit/c1f995226c46c2bba9c5fbf988cf9b247f87adad

A related change talking about llvm 14. https://github.com/apache/arrow/pull/14310

Individua le definizioni CI di jar-build per macOS e Linux e confronta il modo in cui viene selezionato LLVM, quindi esamina arrow/cpp/Brewfile e i dettagli dell’immagine del runner collegata. Il lavoro sarà completato quando le build utilizzeranno una versione coerente di LLVM oppure sarà presente una differenza di versione tra le piattaforme esplicitamente documentata e giustificata.

Scritto dal modello di indicizzazione a partire dal testo della issue.