Specify all NuGet package sources

Azure Pipelines doesn't provide the same defaults as desktop builds, so
builds cannot rely on the packageSources being prepopulated.

Author: sharwell

NuGet.config

@@ -1,6 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="roslyn" value="https://dotnet.myget.org/F/roslyn/api/v3/index.json" />
     <add key="roslyn-analyzers" value="https://dotnet.myget.org/F/roslyn-analyzers/api/v3/index.json" />