Build refs explicitly

.vsts.ix-shared.yml

@@ -14,6 +14,13 @@ steps:
   displayName: Set Version
   condition: eq(variables['system.pullrequest.isfork'], false)
 
+- task: DotNetCoreCLI@2
+  inputs:
+    command: build
+    projects: Ix.NET/Source/refs/**/System.Interactive*.csproj
+    arguments: -c $(BuildConfiguration)   
+  displayName: Build reference assemblies
+
 - task: DotNetCoreCLI@2
   inputs:
     command: pack