Update sample-js-samplebot-win-test.yml for Azure Pipelines

Using 3.1

build/yaml/sample-js-samplebot-win-test.yml

@@ -288,6 +288,11 @@ jobs:
         '-------------'; get-content "$file"; '==================='
     displayName: Create nuget.config for Samples.$(SampleBotName).FunctionalTests.csproj for NuGet.org feed
 
+  - task: UseDotNet@2
+    displayName: 'Use .Net Core sdk 3.1.426'
+    inputs:
+      version: 3.1.426
+
   - task: NuGetCommand@2
     displayName: NuGet restore dotnet Samples.$(SampleBotName).FunctionalTests.csproj
     inputs: