build(deps): bump nixpkgs from a5cc6f2 to 21a67dc

Bumps [nixpkgs](https://github.com/NixOS/nixpkgs) from `a5cc6f2` to `21a67dc`.
- [Commits](NixOS/nixpkgs@a5cc6f2...21a67dc)

---
updated-dependencies:
- dependency-name: nixpkgs
  dependency-version: 21a67dc470149f337cecafbe965d8d252a390518
  dependency-type: direct:production
...

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Author: dependabot[bot]