This documents the fact that nix-build, nix-env, nix-instantiate, and nix-shell accept an extended syntax for their file arguments, including some well-known (but not well documented) aspects, like being able to specify `<nixpkgs>`, but also https:// tarball URLs, `flake:` prefixed flakerefs, and the cursed `channel:` prefixed hardcoded URLs Same thing for the new CLI incoming :) Change-Id: Ib6d68594a16132805ba5d97526e16f7b3633117e