FindJNI: Improve output on macOS if Xcode is not installed

Improve the changes from:

* commit efa0e2ddaf (FindJNI: Update for Xcode greater than 12.1 on macOS,
                     2023-08-06, v3.27.2~5^2)
* commit 597139b853 (FindJNI: Restore support for macOS without Xcode,
                     2023-08-11, v3.27.3~8^2)

to avoid leaking `xcodebuild`'s stderr.

Issue: #26229