fix osx build sdk / runtime.

azure-pipelines.yml

@@ -34,9 +34,17 @@ jobs:
   pool:
     vmImage: 'macOS-10.14'
   steps:
-  - task: DotNetCoreInstaller@0
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core SDK 3.1.101'
     inputs:
-      version: '2.1.403'
+      packageType: sdk
+      version: 3.1.101
+
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core Runtime 3.1.1'
+    inputs:
+      packageType: runtime
+      version: 3.1.1
 
   - task: CmdLine@2
     displayName: 'Install Mono 5.18'