[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

找出 macOS 和 Linux 的 jar-build CI 定義，並比較 LLVM 的選擇方式，接著檢查 arrow/cpp/Brewfile 和連結的 runner-image 詳細資訊。完成的標準是建置使用一致的 LLVM 版本，或平台之間的版本差異已有明確記錄並得到合理說明。

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