flake: prevent the second nixpkgs in lock file

[ilkecan]

Follow the nixpkgs input to avoid having multiple versions of it.

Sanity Checking

• I have updated the changelog as per my changes
• I have tested, and self-reviewed my code
• My changes fit guidelines found in hacking nvf
• Style and consistency
  • I ran Alejandra to format my code (nix fmt)
  • My code conforms to the editorconfig configuration of the project
  • My changes are consistent with the rest of the codebase
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas
  • I have added a section in the manual
  • (For breaking changes) I have included a migration guide
• Package(s) built:
  • .#nix (default package)
  • .#maximal
  • .#docs-html (manual, must build)
  • .#docs-linkcheck (optional, please build if adding links)
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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[NotAShelf]

This is both a good idea and a bad idea at the same time.

NDG is a rust program, and due to the number of Treesitter dependencies it takes quite a while to build. The best approach here is using npins for this (which is what we do for Hjem) but NDG is set to be in Nixpkgs soon, so I'm a little hesitant.

That said, users currently do not have to build NDG--unless they add docs-html package somewhere. @horriblename @Soliprem what do you think?