[hdEmbree] ensure we respect PXR_WORK_THREAD_LIMIT

[pmolodo]

Description of Change(s)

Fixes an issue where pre-oneTBB, threads spawned from the non-root thread (such as the render thread) would not respect PXR_WORK_THREAD_LIMIT

Fixes Issue(s)

• renderThread-spawned processes don't respect PXR_WORK_THREAD_LIMIT

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NOTE: This chain of PRs was made separate from the PR documenting expected behavior:

• [UsdLux] Add explanation of UsdLux quantities and behavior PixarAnimationStudios/OpenUSD#3182

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• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement