github: Migrate away from macos-14 runners

We switched specifically to macos-14 runners in
1a25a0ff74, since the macos-15
runners were notably slower at the time.

The macos-14 runners are being removed now, so we need to switch
to something else.

Before 1a25a0ff74 we occasionally
saw cases where the builds (with macos-latest, resolving to
macos-15 at the time) timed out; we'll have to see if this still
is the case now.

A quick comparison (with a single sample of each), of doing a
single-stage build (without PGO, without LTO) saw macos-14
completing at 3 h 5 m, macos-15 at 4 h 8 m, macos-26 at
4 h 33 m, and xcode-27 at 4 h 7 m - so there's a risk that this
still is 30% slower than macos-14 was.

.github/workflows/build.yml

@@ -259,7 +259,7 @@ jobs:
 
   macos-stage1:
     needs: [prepare]
-    runs-on: macos-14
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v6
       - name: Build
@@ -288,7 +288,7 @@ jobs:
   # Build a cross compiler for macOS, targeting Windows.
   macos:
     needs: [prepare, macos-stage1, linux-profile]
-    runs-on: macos-14
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v6
       - uses: actions/download-artifact@v8