Add global NuGet.config that turns off the user and machine configs

NuGet.config

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <!-- Don't use any higher level config files.
+       Our builds need to be isolated from user/machine state -->
+  <fallbackPackageFolders>
+    <clear />
+  </fallbackPackageFolders>
+  <packageSources>
+    <clear/>
+  </packageSources>
+</configuration>