-BuildManaged -BuildInstallers doesn't work as installer build too early

[ViktorHofer]

When building installers without first building the rest of the repository (managed + native) in a previous build execution, the build fails as the installer build runs too early. This is probably not a problem as that's not a common workflow but with the VMR we need to be able to build the entire repository in one go.

Blocks dotnet/sdk#44828

The solution that I'm thinking of is adding a Traversal project that builds the installer projects and adding that to eng/Build.props at the very end with a BuildInParallel=false metadata so that it runs after everything else. The installer project will still build in parallel.

[ViktorHofer]

cc @wtgodbe

[wtgodbe]

Yeah, our workflow for installers has always been to build them in a separate invocation of build.cmd, after the main repo build. This isn't great, but has never really been a problem since almost nobody ever needs to build the installers locally. I'd be happy to change it in the repo if the VMR solution works, it would simplify our CI/logical model