Explicitly adding 3.1 to test steps. (#6666)

Co-authored-by: Tracy Boehrer <trboehre@microsoft.com>

build/yaml/ci-test-steps.yml

@@ -19,6 +19,14 @@ steps:
     verbose: false
     customCommand: 'install -g @microsoft/botframework-cli@next'
 
+- task: UseDotNet@2
+  displayName: "Install .NET Core 3.1.415"
+  continueOnError: true
+  inputs:
+    packageType: "sdk"
+    version: 3.1.415
+  condition: and(succeeded(), eq(variables['BuildConfiguration'],'Release-Windows'), eq(variables['BuildTarget'],'netcoreapp31'))
+
 - task: DotNetCoreCLI@2
   displayName: 'dotnet test (release) 3.1'
   inputs: