Switch from Pydantic to msgspec

[renan-r-santos]

Fixes #64

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.20%. Comparing base (34a469f) to head (56101d3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pixi_kernel/env.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   71.20%   71.20%           
=======================================
  Files          10       10           
  Lines         257      257           
  Branches       32       32           
=======================================
  Hits          183      183           
  Misses         68       68           
  Partials        6        6           

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