Enable building allConfigurations for corefx in source-build

[weshaggard]

We need to build all packages not just .NET Core app packages when we are in offline source-build mode. However the allconfigurations leg has never worked correctly on non-windows so we need to iterate on it in order to enable.

We need to essentially undo 617ebe0.

[weshaggard]

When we enable this we need to take into account #184

[weshaggard]

@bartonjs this is likely something we will need to do at some point to fully build the packages people depend on upstack.