Clear fallbackPackageFolders in backend/nuget.config

Fixes: ResolvePackageAssets fails with "Unable to find fallback package
folder 'D:\Microsoft Visual Studio\Shared\NuGetPackages'".

That folder comes from a machine-wide Visual Studio installer config
(C:\Program Files (x86)\NuGet\Config\Microsoft.VisualStudio.FallbackLocation.config),
not from anything in this repo — it points at a fallback package cache on
whatever drive VS was originally installed on, and NuGet fails outright if
that path doesn't exist on the current machine. nuget.config already had
<clear/> under packageSources for the same reason (pin restore to nuget.org
only); it just didn't do the equivalent for fallbackPackageFolders. Restore
and build now depend only on this file, not on machine-specific VS install
history.

backend/nuget.config

@@ -4,4 +4,12 @@
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
+  <!-- Machine-wide VS installs (Microsoft.VisualStudio.FallbackLocation.config)
+       can register a fallback package folder on a drive that doesn't exist on
+       every dev machine (e.g. a D:\ that was only ever present on the machine
+       VS was originally installed on). Clearing it here makes restore/build
+       depend only on nuget.org, regardless of that machine-wide config. -->
+  <fallbackPackageFolders>
+    <clear />
+  </fallbackPackageFolders>
 </configuration>