Added UseDotNet task

build/aro/azure-devops/webapp-build.yml

@@ -5,6 +5,12 @@ jobs:
   pool: 
     name: 'Hosted Ubuntu 1604'
   steps:
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core 3.1 SDK'
+    inputs:
+      packageType: sdk
+      version: 3.1.100
+      installationPath: $(Agent.ToolsDirectory)/dotnet
   - task: DotNetCoreCLI@2
     displayName: 'dotnet restore'
     inputs: