chore: bump nixpkgs

[ncfavier]

Bump nixpkgs to get highlighting for opaque and unfolding keywords in --skip-agda builds.

Other notable changes:

• node2nix seems to be deprecated and started failing, so I replaced it with buildNpmPackage (which conveniently does not require storing any generated files in the repository).
• Pandoc 3.6.3 changed the representation of wikilinks.
• Removed some redundant Node.js path overrides.

Tested with and without --skip-agda after removing _build.

(CI dies trying to rebuild everything.)