Try to fix CI

GitHub's macOS images apparently don't have .NET 6 anymore. Skip .NET 6 tests on those.

Also, add x64 macOS CI back, so we're testing both ARM64 macOS and x64 macOS still.

Author: PJB3005

.github/workflows/build.yml

@@ -12,9 +12,15 @@ jobs:
 
     strategy:
       matrix:
-        os: [windows-latest, ubuntu-latest, macos-latest]
+        os: [windows-latest, ubuntu-latest, macos-latest, macos-latest-large]
 
     steps:
     - uses: actions/checkout@v2
-    - name: Test
-      run: dotnet test --configuration Release -v n
+    - name: Test .NET 6
+      # macOS images don't have .NET 6.
+      if: matrix.os != 'macos-latest' && matrix.os != 'macos-latest-large'
+      run: dotnet test --configuration Release --framework net6.0 -v n
+    - name: Test .NET 7
+      run: dotnet test --configuration Release --framework net7.0 -v n
+    - name: Test .NET 8
+      run: dotnet test --configuration Release --framework net8.0 -v n