Fix compare_snippet_output.py by including cv2 in pixi's env (rer…

…un-io#6000)

Before:
```
Traceback (most recent call last):
  File "/home/cmc/dev/rerun-io/rerun/docs/snippets/all/image_advanced.py", line 5, in <module>
    import cv2
ModuleNotFoundError: No module named 'cv2'
```

After: 👍 

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