Export needs to include build commands that use csc.exe

[jaredpar]

Today the export command creates build files that effectively dotnet exec csc.dll for the generated .rsp file. This means it strongly favors .NET Core based compilers vs .NET Framework ones. For the marority of cases the difference is immaterial. There are a few cases though where the runtime matters:

1. Tracking down certain classes of analyzer bugs
2. Performance investigation

The export command should be changed to emit both files to support these scenarios.