feat(step): nix-helper

[dtomvan]

What does this PR do

Add support for viperML/nh. See #1044.

Two decisions I've made upfront in this PR:

1. I added the whoami crate. I found this practical as I don't want to implement these functions myself for multiple platforms. They say they support at least MacOS and Linux, and those are the two Unices that NixOS and nh are targetting anyways, so I think we're good here.
2. I wrote run_nix_helper in a way that never dry-runs the two nix eval commands, which are used to figure out which configs are available in the flake. I added a comment to explain it at line 640, but in short I want to run these in both dry and wet runs so that the outcome of wet runs can be determined in a dry run.

I've run hyperfine on the nix eval command in my not-so-small nix flake (https://github.com/dtomvan/puntbestanden) and found that it doesn't cost a lot of time:

Benchmark 1: nix eval --json ~/puntbestanden#homeConfigurations --apply builtins.attrNames
  Time (mean ± σ):     155.3 ms ±   6.7 ms    [User: 51.8 ms, System: 57.9 ms]
  Range (min … max):   143.0 ms … 170.0 ms    19 runs
 
Benchmark 2: nix eval --json ~/puntbestanden#nixosConfigurations --apply builtins.attrNames
  Time (mean ± σ):     154.9 ms ±   7.5 ms    [User: 50.8 ms, System: 59.3 ms]
  Range (min … max):   135.6 ms … 166.2 ms    18 runs

So together this is only a slowdown of ~310 ms... I hope that's okay even for dry runs.

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• If this PR introduces new user-facing messages they are translated

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

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