ci : update macos-latest* jobs to use macos-latest (#15938)
* ci : update macos-latest* jobs to use macos-latest
This commit updates the jobs that are named macos-latest* to use the
macos-latest label instead explicit versions.
The motivation for this is that there is currently a mixuture of
versions in this workflow and there are jobs that are failing because
they require a newer version.
Refs: https://github.com/ggml-org/llama.cpp/actions/runs/17644792595/job/50140010907#step:5:1759
* ci : add xcodebuild -downloadPlatform iOS command
