Downgrade macos image and specify xcode version

Author: miry

.github/workflows/llvm.yml

@@ -1,6 +1,10 @@
 name: Darwin LLVM build
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - 267-llvm_bin-universal
+      - '*build-llvm*'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -12,12 +16,16 @@ env:
 
 jobs:
   llvm-build:
-    runs-on: macos-13
+    runs-on: macos-11
     defaults:
       run:
         working-directory: ./omnibus
 
     steps:
+      - uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: '13.2.1'
+
       - name: Download Crystal source
         uses: actions/checkout@v4