Migrating NixOS configuration.nix inputs from nix-channel to npins and system.nix

Introduction From the 26.05 release notes: The system.nix file has been added as an alternative entry point to configuration.nix (and flake.nix) that allows configuring NixOS without using nix-channel. Motivated by this thread, which the source of most of what I wrote here, I’ve switched one of my machines from channels to npins and I wrote down the steps for myself. After a few days of updates without issues, I have applied it to another machine successfully and thought it could be useful t...