Fix warning "The OutputPath property is not set for project 'helix.proj'."

[wtgodbe]

Part of #22240

Happens during the Test Helix x64 step. Full warning text below:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(778,5): warning : The OutputPath property is not set for project 'helix.proj'. Please check to make sure that you have specified a valid combination of Configuration and Platform for this project. Configuration='Release' Platform=''. You may be seeing this message because you are trying to build a project without a solution file, and have specified a non-default Configuration or Platform that doesn't exist for this project. [F:\workspace_work\1\s\eng\helix\helix.proj]

@HaoK may know more about this one

[dougbu]

I think we set <OutputPath /> to be intentionally empty to avoid creating an assembly. There are other ways to do that e.g.

<Target Name="CopyFilesToOutputDirectory" />
<Target Name="CoreCompile" />
<Target Name="CreateManifestResourceNames" />

[HaoK]

Here's the arcade issue for this too dotnet/arcade#1605

[dougbu]

Taking this because #22395 works and is less hacky than #22256

[dougbu]

d2b3eec