Use .NET 2.1.5 to create package on Linux build

.vsts/linux-build.yml

@@ -47,6 +47,11 @@ steps:
     testRunner: "VSTest"
     testResultsFiles: "**/*.trx"
 
+- task: DotNetCoreInstaller@0
+  displayName: install dotnet core 2.1.500
+  inputs:
+    version: "2.1.500"
+
 - task: DotNetCoreCLI@1
   displayName: Package Nuget
   inputs: