fix: respect user GL override in render probe

[TATP-233]

Summary

• Fix render_many GL backend resolution so only user-provided MUJOCO_GL overrides are preserved on Linux.
• Avoid treating the module's own import-time fallback (glfw on macOS hosts probing Linux behavior in tests) as an explicit override.
• Restore the expected EGL probe behavior in the macOS unit test path.

Linked Work

• Issue: N/A
• Milestone: N/A

Validation

• make check
• uv run pytest -m "not slow"
• Additional task-specific validation listed below

Commands actually run:

uv run pytest tests/utils/test_render_many.py -q
make check
make test

Impact

• Backend impact: mujoco
• Platform impact: both
• Training effect expected: no

Artifacts

• W&B: N/A
• benchmark result: N/A
• video / screenshot: N/A
• ONNX / checkpoint: N/A

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly