Matrix runner OS

Author: hannesa2

.github/workflows/BuildPR.yml

@@ -9,10 +9,11 @@ on:
 jobs:
   build-objective-git:
     name: Build objective-git
-    runs-on: macOS-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
+        os : [ macOS-12 ]
         xcode: [ Xcode_14.2, Xcode_13.4 ]
     steps:
       - name: ls Xcode