Dogfood multithreaded mode

Real builds surface problems that tests do not. Multithreaded mode already runs in CloudBuild. The next step is the VMR and the repos that build with Arcade, which together cover most of how .NET is built.

## Steps

- [x] Dogfood multithreaded mode in CloudBuild [size S] [Milestone: version 18.11]
- [ ] Dogfood in the VMR [size M] [Milestone: version 18.12]
- [ ] Dogfood in the Arcade repos [size M] [Milestone: version 18.12]

## Stakeholders

The MSBuild team, who need evidence from real builds before enabling `-mt` by default, and the VMR and Arcade repo owners who run those builds.

## Definition of done

The VMR and the Arcade repos build with `-mt`, and the problems those builds surface are fixed or tracked.

Start by reviewing the VMR and Arcade repository build instructions and the existing CloudBuild multithreaded-mode setup. Run the VMR and Arcade builds with `-mt`, then fix or track the problems they surface; done means both build successfully with `-mt`.

Written by the indexing model from the issue text.