Fix pathways v5e perf test

[dipannita08]

Description

The pathways_maxtext_v5e_configs_perf airflow test fails because JAX_PLATFORMS=proxy and JAX_BACKEND_TARGET=... environment vars are missing (Thanks to Luke for rootcausing and suggesting a fix)

Summary of the issue:

• The pathways configs were only being set up in the xpk sub-command and not on-device. (this PR)
• The on-device test on Airflow is not passing the use_pathways flag (THIS PR)

FIXES: b/372059783

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

• python3 benchmarks/benchmark_runner.py on-device --base_output_directory={OUTPUT_PATH} --libtpu_type=maxtext-docker --num_steps=15 --use_pathways=True

Checklist

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• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.