Fix parallel builds

[timcassell]

Fixes #2425

Builds using dotnet sdk 8 or newer use ArtifactsPath to enable safe parallel builds. Builds using older dotnet sdks and wasm and monoaotllvm build sequentially.

It would be nice to enable parallel builds also for wasm and monoaotllvm, but I can't test them on my machine, so I'm not sure why it doesn't work with those toolchains. cc @LoopedBard3 @caaavik-msft