github: Switch macos jobs to macos-14 runners explicitly

For some unknown reason, macos-15 runners (which macos-latest
resolves to currently) seem to be consistently 25-40% slower than
macos-14 runners; the "macos" job (doing an universal binary build
for 2 architectures, with LTO) often take over 5 hours, and have
reached the 6 hour timeout once.

Switch this back to macos-14 to speed it up and reduce the risk of
hitting the timeout.

In the future, if/when the macos-14 runners are phased out, and if
the performance reduction in macos-15 runners remains, we'd need to
switch to building each architecture in a separate job, and merge
them together separately if we still want to provide universal
binaries.

.github/workflows/build.yml

@@ -258,7 +258,7 @@ jobs:
 
   macos-stage1:
     needs: [prepare]
-    runs-on: macos-latest
+    runs-on: macos-14
     steps:
       - uses: actions/checkout@v4
       - name: Build
@@ -287,7 +287,7 @@ jobs:
   # Build a cross compiler for macOS, targeting Windows.
   macos:
     needs: [prepare, macos-stage1, linux-profile]
-    runs-on: macos-latest
+    runs-on: macos-14
     steps:
       - uses: actions/checkout@v4
       - uses: actions/download-artifact@v4