Clear fallback package folders to prevent VS shared nuget cache from providing Microsoft.Windows.SDK.BuildTools since it doesn't include the origin .nupkg which is required for the C++ VSIX (#3596)

NuGet.config

@@ -12,4 +12,7 @@
   <disabledPackageSources>
     <clear />
   </disabledPackageSources>
+  <fallbackPackageFolders>
+    <clear />
+  </fallbackPackageFolders>
 </configuration>

eng/common/nuget.config

@@ -15,4 +15,7 @@
   <disabledPackageSources>
     <clear />
   </disabledPackageSources>
+  <fallbackPackageFolders>
+    <clear />
+  </fallbackPackageFolders>
 </configuration>