NuGet.Client does not support building with source-build args or --source-build flag

NuGet.Client doesn't have much of the arcade infrastructure, so it doesn't run a source-build build when provided with the standard source-build flags or the new --source-build flag.

At the very least, we should make `./build.sh --source-build` map to `./eng/source-build/build.sh`, which is the command to successfully build the source-build configuration today.