Poison leaks for NETStandard 2.1 and Microsoft.NET.Build.Extensions

[mthalman]

Poison log: poison-usage.zip

Example build (internal link)

Seems to have been caused by either dotnet/dotnet#812 or dotnet/dotnet#798

[MichaelSimons]

Another piece to investigate is why the build is failing. The build should be creating a poison report and not failing the build. The poison test is what should actually fail the build. Quickly looking at the leaks, these appear to be what is in the baseline as allowed prebuilts.

[mthalman]

Yep, the leak is matching the baseline so this is expected. It's failing the build because of dotnet/dotnet#798 which is now treating warnings as errors.