build(deps): bump nixpkgs-unstable from b3da656 to c6e5ca3

Bumps [nixpkgs-unstable](https://github.com/NixOS/nixpkgs) from `b3da656` to `c6e5ca3`.
- [Commits](NixOS/nixpkgs@b3da656...c6e5ca3)

---
updated-dependencies:
- dependency-name: nixpkgs-unstable
  dependency-version: c6e5ca3c836a5f4dd9af9f2c1fc1c38f0fac988a
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]