Allow worker nodes to accept work from any active build, not just 'lock' to a single build at a time

### Summary

Now that we have the safety guarantees that the Multithreaded APIs (and marker interfaces) provide, we can consider an exciting feature - allowing worker nodes to do work across multiple active builds, taking work from whichever builds are active instead of being constrained to a single logical build for the duration of that build.

Before these APIs, this was too risky because Tasks couldn't be trusted to be stateless - but with the APIs (and some additional documentation clarifying how stateful constructs like C# statics should be used safely) we can experiment with this possibility, which should help prevent the thundering-herd problem of multiple concurrent builds spawning NUMPROCS nodes that we have today.

### Background and Motivation

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### Proposed Feature

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### Alternative Designs

_No response_