Nix config: add extra newlines after the initial comment

[grahamc]

Otherwise the extra-config option will get swallowed.

Closes #717

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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