fix(CI unit tests, cute_dsl, spark): set USER env var before torch._dynamo import for unmapped UIDs (#3314)

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## 📌 Description

Summary
Follow-up to #3290. The lazy wrapper in #3290 fixed the import-time
crash (KeyError: 'getpwuid(): uid not found: 996') for all tests by
deferring torch._dynamo.disable from module load time to first call.
However, test_b12x_fused_moe still hit the same error at runtime because
it's the only test that actually calls current_cuda_stream(), which
triggers the lazy wrapper and imports torch._dynamo.

The root cause is that torch._dynamo initialization calls
getpass.getuser(), which falls back to pwd.getpwuid(os.getuid()) when no
username env var is set. In .dlfw_runners CI containers (Spark, 5090,
Thor) running with -u $(id -u):$(id -g), the host UID (996) has no
/etc/passwd entry.

This fix sets a fallback USER env var before the torch._dynamo import so
getpass.getuser() returns from its env-var check and never reaches the
failing getpwuid call.

## 🔍 Related Issues

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## Summary by CodeRabbit

* **Bug Fixes**
* Improved reliability in containerized environments by enhancing how
user environment information is detected, preventing failures when
running in containers.

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Co-authored-by: Alex Yang <aleyang@nvidia.com>