When a NixOS system uses flakes, i.e., /etc/nixos/flake.nix exists, it
is impossible to use nixos-rebuild to build a pre-flake
configuration.nix. Of course, one can directly use nix command to
build the configuration, but not everybody remembers the correct nix
options to do that.
With the new option, it is possible to build a pre-flake configuration
with command like this:
nixos-rebuild build-vm -I nixos-config=./vm.nix --no-flake
The option might be useful for people following older pre-flake
tutorials on a flake-based system.
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