[CI]: temporarily be explicit about using the macos-12 image (#7882)

* temporarily be explicit about using the macos-12 image
This is driven by the changes suggested here: https://github.com/actions/runner-images/issues/9262
We need to update the mac build scripts to install miniforge ourselves.
<sigh>

* lock the compiler version too
19.1 seems to cause problems with the libcffi code

azure-pipelines.yml

@@ -35,9 +35,9 @@ jobs:
 - job: macOS_x64
   timeoutInMinutes: 120
   pool:
-    vmImage: macos-latest
+    vmImage: macos-12
   variables:
-    compiler: clangxx_osx-64
+    compiler: clangxx_osx-64=18.1
     compiler_version: 12.0
     boost_version: 1.82.0
     number_of_cores: sysctl -n hw.ncpu
@@ -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