VMR Builds are slow on release branches due to SKU difference

[mmitche]

Arcade will automatically switch over to a servicing pool when running on release/*. This halves the number of cores that we run on, which presents a significant regression in our ability to get builds out the door.

This might be okay for actual servicing? Maybe not. Either way, I think that for now, we should conditionalize so that preview branches do not hit the servicing pools.

[mmitche]

Resolved temporarily with dotnet/sdk#48694. Moving to the p5 critical so that we make a real fix by then.