temporarily be explicit about using the macos-12 image

This is driven by the changes suggested here: actions/runner-images#9262
We need to update the mac build scripts to install miniforge ourselves.
<sigh>

azure-pipelines.yml

@@ -35,7 +35,7 @@ jobs:
 - job: macOS_x64
   timeoutInMinutes: 120
   pool:
-    vmImage: macos-latest
+    vmImage: macos-12
   variables:
     compiler: clangxx_osx-64
     compiler_version: 12.0
@@ -106,7 +106,7 @@ jobs:
 - job: macOS_x64_java
   timeoutInMinutes: 90
   pool:
-    vmImage: macos-latest
+    vmImage: macos-12
   variables:
     compiler: clangxx_osx-64
     compiler_version: 15.0