feat: option to auto-format Nix output using nixfmt

[aksiksi]

TODO:

• Figure out how to get auto-format test to pass during Flake build. For some reason, nixfmt isn't being included during Flake build.

[codecov]

Codecov Report

Attention: Patch coverage is 38.09524% with 26 lines in your changes missing coverage. Please review.

Project coverage is 71.83%. Comparing base (e5f3568) to head (31d891f).

Files	Patch %	Lines
main.go	0.00%	12 Missing ⚠️
helpers.go	47.36%	6 Missing and 4 partials ⚠️
nix.go	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   73.88%   71.83%   -2.06%     
==========================================
  Files           6        6              
  Lines         877      710     -167     
==========================================
- Hits          648      510     -138     
+ Misses        193      158      -35     
- Partials       36       42       +6     

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