test: override XDG_DATA_HOME so menuinst writes shortcuts

[hunger]

…ide the per-test data_home

How Has This Been Tested?

The unit tests

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

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.

[hunger]

I had expected this to be the first that went in: It's a pretty obvious omission.