Run a command in a shell with packages from pinned nixpkgs
NIX_BUILD_SHELL=bash nix-shell - --run "npins update" < <(echo "{sources ? import ./npins, pkgs ? import sources.nixpkgs {}}: pkgs.mkShell {buildInputs=[pkgs.npins];}")
I have yet to find a way to start a shell using this method. It seems like nix-shell thinks it's not "interactive" because the "shell.nix" is passed via stdin.