Add workflow to update pixi lock files

[ColmTalbot]

This workflow updates lock files weekly using pixi and creates a pull request with the changes. It can also be manually triggered.

[ColmTalbot]

I think this can't actually be used/meaningfully tested until it is on main.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.75%. Comparing base (c892d2b) to head (ab8c92d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1142      +/-   ##
==========================================
- Coverage   71.79%   71.75%   -0.04%     
==========================================
  Files          86       86              
  Lines       15400    15400              
  Branches     2343     2343              
==========================================
- Hits        11057    11051       -6     
- Misses       3638     3641       +3     
- Partials      705      708       +3     

Flag	Coverage Δ
python311	71.72% <ø> (-0.04%)	⬇️
python312	71.72% <ø> (-0.04%)	⬇️
python313	71.72% <ø> (-0.01%)	⬇️

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