Use macOS Big Sur and Xcode 13 for CI

Author: trflynn89

build/ci/macos.yml

@@ -9,7 +9,7 @@ jobs:
       value: $(Build.SourcesDirectory)/ccache
 
     pool:
-      vmImage: macOS-10.15
+      vmImage: macOS-11
 
     steps:
     - script: |
@@ -25,7 +25,7 @@ jobs:
         arch: ${{ parameters.arch }}
 
     - script: |
-        echo "##vso[task.setvariable variable=config]mode=${{ parameters.mode }} cacher=ccache"
+        echo "##vso[task.setvariable variable=config]mode=${{ parameters.mode }} cacher=ccache XCODE=/Applications/Xcode_13.0.app/Contents/Developer"
       displayName: 'Configure'
 
     - ${{ if eq(parameters.mode, 'debug') }}: