[infra] Builds can use multiple pools for even more parallelism

We currently have a 400 concurrent build limit per private pool. We have 700 projects and 3 kinds of builds (fuzzing, coverage, introspector)
We can pretty easily use different pools for different kinds of build, this should make trial builds even faster!

The issue names no files, tests, or entry points. Start by locating the private-pool and build scheduling configuration, then determine how fuzzing, coverage, and introspector builds are assigned to pools. Done means the three build kinds can use separate pools and trial builds gain the intended parallelism.

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