Refactor eng\validate-sdk.ps1

[JohnTortugo]

We should consider changing the approach used by [validate-sdk.ps1](https://github.com/dotnet/arcade/blob/65820ecd54acd04013f95f2635c284459ba2c273/eng/validate-sdk.ps1#L86) where it moves the whole artifacts\ folder. This is causing a lot of interference with things that run after the script.

@chcosta @riarenas @alexperovich

[chcosta]

Agreed, there are already some issues tracking this type of change

• https://github.com/dotnet/arcade/blob/master/eng/validate-sdk.ps1#L83
• https://github.com/Microsoft/azure-pipelines-yaml/blob/master/design/container-steps.md

[JohnTortugo]

@riarenas just FYI, this is the reason we have those copy files around in PushToAzureDevOpsArtifacts: https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Build.Tasks.Feed/src/PushToAzureDevOpsArtifacts.cs#L75

[chcosta]

This is removed in #3530 where we just do the "validate-sdk" type work in a different stage and we no longer copy the artifacts folder around.

[JohnTortugo]

That's all I wanted to hear :-)