Installing only a single package from `unstable`

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@dalto Is there a way to add the unstable nix channel within one’s configuration.nix ?

@fosskers You had a perfectly valid question there, I see no reason to withdraw it. The answer can be found elsewhere, but it is good to have multiple references for visibility. Unfortunately the prominent NixOS Wiki search result does not even feature it. Really it is a combination of that and the example on pinning nixpkgs:

# configuration.nix
{ config, pkgs, ... }:
let
  unstable = import
    (builtins.fetchTarball https://github.com/nixos/nixpkgs/tarball/<branch or commit>)
    # reuse the current configuration
    { config = config.nixpkgs.config; };
in
{
  environment.systemPackages = with pkgs; [
    nginx
    unstable.certbot
  ];
}

You should of course specify a hash, and probably a derivation name, my example is simplified for readability. Check the Nix manual for builtin function specs.

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