Build TaskAnalyzers project in source-build mode

We plan to add a package dependency on the TaskAnalyzers package in Microsoft.Build.Framework. That has shipping impact. The TaskAnalyzers package is currently not building in source-build mode: https://github.com/dotnet/msbuild/blob/main/MSBuild.SourceBuild.slnf

@baronfel previously tried to enable it but that then introduced prebuilts in the VMR: https://github.com/dotnet/msbuild/commit/f795830904f855e65ef5bb47cc33bba363b4b7a5

SB validates that we a consistent set of dependencies between the MSFT and SB builds. Even though we aren't shipping the Microsoft.Build.Framework package produced by source-build, we must avoid differences between the two build modes when possible. If that's not possible, we would need an exception in the VMR source-build tests.

This task tracks start building the TaskAnalyzer under source-build (by adding the project to the slnf) and avoiding the prebuilts that previously showed up when building in the VMR.

aspnetcore had to solve this as well: https://github.com/dotnet/dotnet/blob/89fcf59fb34c25e7fa2afd3f214ba8df93b30bce/repo-projects/aspnetcore.proj#L66-L82

Reach out to @mthalman who introduced that infrastructure, if you need help.