refactor(nix): clean flake

[drainpixie]

Relevant issues

Brief

Clean up flake.nix

Changes

• Split logic across multiple files in the nix/ folder
• Split helpers in a separate file
• Use lib.getExe{,'} over {pkgs,derivation}/bin/{name}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.56%. Comparing base (f6b9f69) to head (0403ff6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1464   +/-   ##
=======================================
  Coverage   92.56%   92.56%           
=======================================
  Files         100      100           
  Lines        1022     1022           
=======================================
  Hits          946      946           
  Misses         76       76           

Flag	Coverage Δ
macos-latest	91.97% <ø> (ø)
py3.10	92.56% <ø> (ø)
py3.11	92.56% <ø> (ø)
py3.12	92.56% <ø> (ø)
py3.13	91.97% <ø> (ø)
py3.14	91.97% <ø> (ø)
ubuntu-latest	91.97% <ø> (ø)
windows-latest	92.56% <ø> (ø)

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