gh-140189: Use macos-14 runner for iOS CI tests (#142444)

Switch to the macos-14 runner for iOS CI, working around some instability 
in the macos-15 runner.

Author: freakboy3742

.github/workflows/build.yml

@@ -392,7 +392,7 @@ jobs:
     needs: build-context
     if: needs.build-context.outputs.run-ios == 'true'
     timeout-minutes: 60
-    runs-on: macos-15
+    runs-on: macos-14
     steps:
       - uses: actions/checkout@v4
         with:
@@ -405,10 +405,10 @@ jobs:
       # https://github.com/actions/runner-images/issues/12751.
       - name: Select Xcode version
         run: |
-          sudo xcode-select --switch /Applications/Xcode_16.4.app
+          sudo xcode-select --switch /Applications/Xcode_15.4.app
 
       - name: Build and test
-        run: python3 Apple ci iOS --fast-ci --simulator 'iPhone 16e,OS=18.5'
+        run: python3 Apple ci iOS --fast-ci --simulator 'iPhone SE (3rd generation),OS=17.5'
 
   build-wasi:
     name: 'WASI'