chore: switch to nur

[2bndy5]

Since everything else in this org now uses nur also, I'm no longer keeping just as this repo's task runner.

Using nushell script also allows me to do computational things that I couldn't in just because the shell used by just was specific to the OS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e7b29c1) to head (0b65d21).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         3250      3250           
=========================================
  Hits          3250      3250           

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