Update dotnetcore.yml

Author: darkl

.github/workflows/dotnetcore.yml

@@ -12,7 +12,9 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '9.0.x'
+        dotnet-version: |
+            '9.0.x'
+            '3.1.x'
     - name: Build with dotnet
       working-directory: src/netstandard/
       run: dotnet build --configuration Release