chore: Fix deprecation warnings in pixi.toml

[borchero]

Motivation

See e.g. https://github.com/Quantco/dataframely/actions/runs/18784880084/job/53600418529#step:3:195

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (91961d5) to head (e01e670).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #199   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         2996      2996           
=========================================
  Hits          2996      2996           

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