chore: Use pixi to manage test environment

[effigies]

Accidentally started on this in #988, so might as well finish it.

Builds on fmriprep's base image... There's no reason to duplicate that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.20%. Comparing base (03d056c) to head (696beb9).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #990   +/-   ##
=======================================
  Coverage   71.20%   71.20%           
=======================================
  Files          89       89           
  Lines        8578     8578           
  Branches      931      931           
=======================================
  Hits         6108     6108           
  Misses       2238     2238           
  Partials      232      232           

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