ci : revert back to macos-13 for macOS-latest-cmake-x64 (#16040)

This commit reverts the change of the runs-on parameter for the
macOS-latest-cmake-x64 job back to macos-13 that was make in
Commit 51abc96bdc52ba8cd6ad78dcf12ed9a041d7b442 ("ci : update
macos-latest* jobs to use macos-latest (#15938)").

The motivation for this is that using macos-latest will cause an ARM
based runner to be used, and not an x64 based runner.

Refs: https://github.com/ggml-org/llama.cpp/pull/15938#issuecomment-3300805127

.github/workflows/build.yml

@@ -97,7 +97,7 @@ jobs:
           ctest -L 'main|curl' --verbose --timeout 900
 
   macOS-latest-cmake-x64:
-    runs-on: macos-latest
+    runs-on: macos-13
 
     steps:
       - name: Clone