dotnet/sdk's non-incremental build is slow

[ViktorHofer]

1. sdk restores runtime pack for 6.0, 7.0, 8.0 and 9.0 which I never need as I don't run the repo tests. It would be great to just-in-time bring those in when tests are run.
2. sdk restores and builds test projects. It would be great to be able to only build the product, like in other repositories, i.e. runtime. slnx might help here. If slnx would allow to reference other .slnx files, there could be a product .slnx file and a tests .slnx file. cc @baronfel

Due to the two above problems, my sdk build (build.cmd) takes ~40-50% longer than necessary.