Generate tarball smoke-test nuget.config

eng/SourceBuild.Tarball.targets

@@ -25,6 +25,8 @@
     Project="SourceBuild.Tarball.DestructiveCleanup.targets"
     Condition="'$(DestructiveIntermediateClean)' == 'true'"/>
 
+  <UsingTask AssemblyFile="$(XPlatSourceBuildTasksAssembly)" TaskName="RemoveInternetSourcesFromNuGetConfig" />
+
   <Target Name="GetTarballDirProps">
     <PropertyGroup>
       <TarballRootDir>$([MSBuild]::NormalizeDirectory('$(TarballRoot)'))</TarballRootDir>
@@ -151,23 +153,39 @@
         Include="$(ProjectDir)support/tarball/build.sh"
         RelativeDestination="build.sh" />
 
-      <!--
-        When building an internal build, the main smoke-test nuget config has authenticated sources
-        in it to generate a full set of smoke-test-prereqs. This isn't necessary in the tarball
-        because we have those smoke-test-prereqs, but we hit authentication errors nonetheless. Copy
-        a trimmed down nuget.config in this case.
-        TODO: Automatically trim down the main nuget.config.
-      -->
-      <TarballCopyFile
-        Condition="Exists('$(ProjectDir)support\tarball\smoke-testNuGet.Config')"
-        Include="$(ProjectDir)support\tarball\smoke-testNuGet.Config"
-        RelativeDestination="smoke-testNuGet.Config" />
-
       <!-- Setup package version props to include both source-built and running PackageVersions.props -->
       <TarballCopyFile
         Include="$(ProjectDir)support\tarball\PackageVersions.props"
         RelativeRoot="artifacts/obj/$(Platform)/Release/" />
     </ItemGroup>
+
+    <!--
+      When building an internal build, the main smoke-test nuget config has authenticated sources in
+      it to generate a full set of smoke-test-prereqs. We need to get rid of these internal feeds in
+      the tarball to avoid hitting authentication errors. (In the tarball, we have those
+      smoke-test-prereqs in a local feed, so this is fine.)
+
+      Use the nuget trimming task to create a tarball nuget.config without the internal feeds, for
+      this case. Importantly, this leaves public feeds alone: when running a public servicing build,
+      we need to use them for runtime packs, and the smoke-test-prereqs tarball won't contain them.
+    -->
+    <PropertyGroup>
+      <FilteredSmokeTestNuGetConfig>$(ArtifactsDir)support\smoke-testNuGet.Config</FilteredSmokeTestNuGetConfig>
+    </PropertyGroup>
+
+    <Copy
+      SourceFiles="$(ProjectDir)smoke-testNuGet.Config"
+      DestinationFiles="$(FilteredSmokeTestNuGetConfig)" />
+
+    <RemoveInternetSourcesFromNuGetConfig
+      NuGetConfigFile="$(FilteredSmokeTestNuGetConfig)"
+      OfflineBuild="false" />
+
+    <ItemGroup>
+      <TarballCopyFile
+        Include="$(FilteredSmokeTestNuGetConfig)"
+        RelativeDestination="smoke-testNuGet.Config" />
+    </ItemGroup>
   </Target>
 
   <Target Name="AddTarballPackages">