Don't include the version number in the nix config, for reproducibility

[grahamc]

I just installed Nix about a hundred times, and got the same hash for the config every time. This was largely because we updated the nix-config-parser crate to use an ordered HashMap, instead of the regular HashMap. The remaining variability in here will be from the version number in the comment, and any custom configuration the user has specified. This PR fixes that :).

Closes #562
Closes #580

Description

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
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