ci : use macos-latest for arm64 webgpu build (#16029)
This commit updates the runs-on field for the macOS arm64 webgpu build
job to use macos-latest instead of just latest.
The motivation for this is that this job can wait for a runner to pick
up the job for a very long time, sometimes over 7 hours. This is an
attempt to see if this change can help reduce the wait time.
Refs: https://github.com/ggml-org/llama.cpp/actions/runs/17754163447/job/50454257570?pr=16004
