In 9.0.0-preview.4, I noticed that this explicit restore operation:

https://github.com/dotnet/dotnet/blob/7b92d4092abc8b04f79a9412a712257966724494/src/aspnetcore/eng/Tools.props#L7

uses $HOME/.packages. This seems like it's probably unintentional. Adding the following causes it to use the inner source build package cache, which is what I would expect it to do:

        Properties="NUGET_PACKAGES='$(CurrentRepoSourceBuildPackageCache)'"