Globally applying overlays onto Nixpkgs

These days, it is fashionable to have the Nixpkgs for your NixOS system managed alongside its configuration, pinned by some tool of choice. In Pinning NixOS with npins, this time without flakes for real , I've talked about how I want the same Nixpkgs to be used for both my system configuration as well as all my ad-hoc nix-shell s. However, there is a key distinction between any import <nixpkgs> {}…