Update CoreFx's NuGet.config

Author: crummel

repos/corefx.proj

@@ -55,6 +55,7 @@
 
     <CleanCommand>$(ProjectDirectory)/clean$(ShellExtension)</CleanCommand>
 
+    <NuGetConfigFile>$(ProjectDirectory)/NuGet.config</NuGetConfigFile>
     <EnvironmentRestoreSources>$(SourceBuiltPackagesPath)</EnvironmentRestoreSources>
     <EnvironmentRestoreSources Condition="'$(OfflineBuild)' == 'true'">$(EnvironmentRestoreSources)%3B$(ReferencePackagesDir)%3B$(PrebuiltPackagesPath)</EnvironmentRestoreSources>
     <EnvironmentRestoreSources Condition="'$(OfflineBuild)' != 'true'">$(EnvironmentRestoreSources)%3Bhttps://api.nuget.org/v3/index.json</EnvironmentRestoreSources>