work: move work thread limit within namespace

[rpoyner-tri]

Description of Change(s)

Puts the PXR_WORK_THREAD_LIMIT symbol within the PXR_NAMESPACE.

Other TF_DEFINE_ENV_SETTING() invocations are within the namespace; this is the only one I've found that is not.

Fixes Issue(s)

Partial fix for #3279.

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-10095

[jesschimein]

/AzurePipelines run

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[nvmkuruc]

This seems like simple and helpful fix.

@pmolodo FYI-- if this gets accepted, it may affect #3370

[pmolodo]

This seems like simple and helpful fix.

@pmolodo FYI-- if this gets accepted, it may affect #3370

Actually, released some changes to #3370 so that it won't be affected - PXR_WORK_THREAD_LIMIT isn't exported as part of the interface, so I had to retrieve the env var by name (via Tf_GetEnvSettingByName, which despite the Tf_ prefix, is exported).