docs: Extend setup-pixi documentation with working-directory

[pavelzw]

prefix-dev/setup-pixi#236, prefix-dev/setup-pixi#247

Description

Fixes #{issue}

How Has This Been Tested?

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• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[pavelzw]

why is the lint step failing?