ci : use macos-latest for arm64 webgpu build (ggml-org#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

Author: danbev

.github/workflows/build.yml

@@ -138,7 +138,7 @@ jobs:
           ctest -L main --verbose --timeout 900
 
   macOS-latest-cmake-arm64-webgpu:
-    runs-on: latest
+    runs-on: macos-latest
 
     steps:
       - name: Clone