Deduplicate GR00T PYTHONPATH bootstrap logic

[alexmillane]

Summary

Deduplicate GR00T PYTHONPATH bootstrap logic

Detailed description

• Duplicate GR00T PYTHONPATH bootstrapping code existed in policy_runner.py and conftest.py, risking divergence.
• Extracted into a shared ensure_groot_deps_in_path() in isaaclab_arena_gr00t/utils/groot_path.py with a reexec_argv parameter to support both script and pytest re-exec. Updated both call sites to use it.
• Add TODO to remove it in v0.3

[xyao-nv]

Since #523 stalls, let's merge this.