github: Switch back to macos-latest for the runners

Currently, "macos-latest" gets "macos-14", while there also are
"macos-15" runners available. Switch back to the latest variant
in preparation for that being updated at some point.

This reverts 7bd4b28.

Author: mstorsjo

.github/workflows/build.yml

@@ -189,7 +189,7 @@ jobs:
   macos:
     if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [prepare]
-    runs-on: macos-14
+    runs-on: macos-latest
     steps:
       - uses: actions/checkout@v4
       - name: Build