clean up nix expressions

[moni-dz]

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Description

tldr

• update the lockfile, it's months old already
• remove the old default* attributes
• refactor some hacks like that pkgs attribute
• actually make nix flake check happy

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Checklist

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• [ ] All widgets I've added are correctly documented.
• [ ] I added my changes to CHANGELOG.md, if appropriate.
• [ ] The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• [ ] I used cargo fmt to automatically format all code before committing