Investigate Previously Source-Built Poison Leaks

[ellahathaway]

Related to dotnet/dotnet#1762

We started detecting poison leaks of PSB artifacts. The poison baseline was updated in dotnet/dotnet#1762 with these leaks, but the leaks still need to be investigated.

[MichaelSimons]

Let's use #5308 to track this. No sense in having multiple issues for the same work.