Install .NET 3.1 as it is required by the vPack task (#17600)

tools/releaseBuild/azureDevOps/templates/vpackReleaseJob.yml

@@ -16,6 +16,13 @@ jobs:
   - checkout: self
     clean: true
 
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core sdk'
+    inputs:
+      packageType: sdk
+      version: 3.1.x
+      installationPath: $(Agent.ToolsDirectory)/dotnet
+
   - template: ./SetVersionVariables.yml
     parameters:
       ReleaseTagVar: $(ReleaseTagVar)