Set PXR_WORK_THREAD_LIMIT=1 for kitless rendering correctness tests

[mataylor-nvidia]

Limits OpenUSD's work-thread pool to a single thread for all kitless rendering tests to improve determinism in golden-image comparisons.

Description

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Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

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Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

This PR limits OpenUSD worker concurrency for kitless rendering tests. The main change is:

• Sets PXR_WORK_THREAD_LIMIT=1 in the shared autouse fixture before each rendering test.

Confidence Score: 5/5

This looks safe to merge.

• The autouse fixture sets the worker limit before the test bodies perform their lazy rendering imports and initialize simulation state.
• Pytest restores the previous environment value after each test.
• No blocking issues were found in the changed code.

Important Files Changed

Filename	Overview
source/isaaclab_tasks/test/rendering_test_utils.py	Adds a function-scoped environment override before kitless tests initialize OpenUSD and simulation state.

_{Reviews (1): Last reviewed commit: "Set PXR_WORK_THREAD_LIMIT=1 for kitless ..." | Re-trigger Greptile}

[pbarejko]

This needs an explanation a comment why this is here

[mataylor-nvidia]

will do

[AntoineRichard]

@mataylor-nvidia can we add a TODO for when this can be removed? Do we expect a drop in start-up performance here?