Use global.json https://docs.microsoft.com/en-us/dotnet/core/tools/global-json to fix tooling version to 3 (RC of VS2019 uses only RTM SDKs)

global.json

@@ -0,0 +1,5 @@
+{
+  "sdk": {
+    "version": "3.0.100-preview3-010431"
+  }
+}