github: Switch the macos build job to macos-14

Currently, the "macos-latest" runners aren't really the latest
version - on the contrary, they're running the oldest HW/OS.

Currently, there are macos-12, macos-13 and macos-14, and macos-latest
are the macos-12 ones.

The macos-14 ones run on Apple Silicon.

Previously, the macos-12 runners have usually managed to run the build
job in less than 3 hours. However, lately, the jobs often end up
taking much longer, over 5 hours, and have ended up exceeding the
6 hour timeout occasionally, even if they still also some times run
in the expected time of less than 3 hours.

Switch to the faster macos-14 runners (which can complete the build
job in around 1 h 40 min), to avoid these timeouts.

.github/workflows/build.yml

@@ -187,7 +187,7 @@ jobs:
   macos:
     if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [prepare]
-    runs-on: macos-latest
+    runs-on: macos-14
     steps:
       - uses: actions/checkout@v4
       - name: Build