CI: use macOS 12 (instead of 13/14/latest)

A macOS job started failing for no reason so maybe it's
because macOS-latest now maps to macOS14 (which has a
different CPU, AFAIU) or macOS13. See:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

.github/workflows/CI.yml

@@ -91,7 +91,7 @@ jobs:
         git diff --exit-code
 
   macOS--dotnet6-and-mono:
-    runs-on: macOS-latest
+    runs-on: macOS-12
     steps:
     - uses: actions/checkout@v1
     - name: Setup .NET SDK 6.0.x
@@ -120,7 +120,7 @@ jobs:
       run: ./compileFSharpScripts.fsx
 
   macOS--mono:
-    runs-on: macOS-latest
+    runs-on: macOS-12
     steps:
     - uses: actions/checkout@v1
     - name: HACK to emulate dotnet uninstall