Pinning NixOS with npins, this time without flakes for real

Last year, Jade posted Pinning NixOS with npins, or how to kill channels forever without flakes . In short, what we want is: Get rid of channels (as in nix-channel , and not any other overloaded meanings of that word) as a source of impurity. Have one blessed Nixpkgs pinned with npins being used for the system's configuration as well as all <nixpkgs> invocations system-wide. The solution should…