--file for Classic Nix

[toastal]

Is your feature request related to a problem? Please describe.

If not buying into experimental flakes, there doesn’t seem to be an option to use system-manager.

Describe the solution you'd like

Similar to nix run --file …, nixos-rebuild --file …, & home-manager --file …, allow users to pass file that directly build a system.

Describe alternatives you've considered

Enabling experimental flake support on this small setup of mine, & adding a dummy flake.nix that just imports the file. This option should be even simpler since it explicitly says what to build instead of needing to evaluate a flake.

Additional context

[eureka-cpu]

There's a plan to allow for a non-flake setup, but I ran out of time to work on it as other things took priority ):