Enable use of SignTool using .NET Core MSBuild

[mmitche]

See https://microsoft-my.sharepoint.com/:w:/p/mmitche/ERcEQy4Xex9CpRyTiGx9HrkBqf4iGDjDjwSk98QsczcROw?e=A8pF4J for design info.

Microbuild signing now supports .NET Core MSBuild and signing on Mac and Linux. This issue covers altering signtool to execute on core (I think it may already do so), but also using .NET Core MSBuild to submit to MicroBuild.

See https://devdiv.visualstudio.com/DevDiv/_wiki/wikis/DevDiv.wiki/12267/ESRP-Signing-in-Mac-or-Linux-Pipelines for more info.