feat: add support for persistent Pixi shells

[renan-r-santos]

prefix-dev/pixi#1080 introduced a feature that allows for a persistent manifest-path when operating within a Pixi shell.
This PR supports this by always passing --manifest-path when invoking pixi.
@abkfenris in case you want to take a look.

[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 63.26%. Comparing base (f0a85f0) to head (8550131).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/pixi_kernel/__main__.py	52.94%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   62.88%   63.26%   +0.37%     
==========================================
  Files           4        4              
  Lines          97       98       +1     
  Branches       14       15       +1     
==========================================
+ Hits           61       62       +1     
  Misses         32       32              
  Partials        4        4              

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[abkfenris]

Thanks! This looks like it does the trick in a quick test of the environment I'm trying to move from miniconda/nb-conda-kernels.