https://nixos.org/manual/nixos/stable/index.html#sec-changing-config

nixos-rebuild
    build
    test
    ...
    -I nixos-config=path/to/your/configuration.nix
    switch [--upgrade]
        sudo nixos-rebuild -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/release-20.09.tar.gz switch
        ! to upgrade release
        -p p1
            to create profile "p1"