CI: Pin Xcode version to 15.3, as default 15.0.1 is broken

Not needed in later Godot branches as we merged workarounds for the 15.0/15.0.1
broken linker, but there's too many commits to track down and backport with
care to avoid breaking Xcode 14 support too.

.github/workflows/macos_builds.yml

@@ -35,6 +35,11 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - name: Setup Xcode
+        uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: '15.3'
+
       - name: Setup Godot build cache
         uses: ./.github/actions/godot-cache
         with: