docs: simplify non-flakes instructions

Author: fricklerhandwerk

docs/howtos/use-without-flakes.md

@@ -1,49 +1,39 @@
 # Use without flakes
 
-While `nixos-anywhere` is designed to work optimally with Nix Flakes, it also
-supports the traditional approach without flakes. This document outlines how to
-use `nixos-anywhere` without relying on flakes. You will need to
-[import the disko nixos module](https://github.com/nix-community/disko/blob/master/docs/HowTo.md#installing-nixos-module)
+First,
+[import the disko NixOS module](https://github.com/nix-community/disko/blob/master/docs/HowTo.md#installing-nixos-module)
 in your NixOS configuration and define disko devices as described in the
 [examples](https://github.com/nix-community/disko/tree/master/example).
 
-## Generate Required Store Paths
-
-Before you can use `nixos-anywhere` without flakes, you'll need to manually
-generate the paths for the NixOS system toplevel and disk image. The paths are
-generated using `nix-build` and are necessary for executing `nixos-anywhere`.
-
-### Generating Disk Image without Dependencies:
+Let's assume that your NixOS configuration lives in `configuration.nix` and your
+target machine is called `machine`.
+
+```nix
+# configuration.nix
+{
+  pkgs ? import (fetchTarball channel:nixos-24.05) {};
+}:
+{
+  config = { config, pkgs, ... }: { /* ... */ };
+  machine = pkgs.nixos config;
+}
+```
 
-To generate the disk image without dependencies, execute:
+Generate the disk formatting script:
 
 ```bash
-nix-build -I nixos-config=/etc/nixos/configuration.nix -E '(import <nixpkgs/nixos> {}).config.system.build.diskoNoDeps'
+disko=$(nix-build configuration.nix -A machine.config.system.build.disko' --no-out-path)
 ```
 
-This will output a script path in `/nix/store` that will format your disk. Make
-note of this path for later use.
-
-### Generating NixOS System Toplevel:
-
-Execute the following command to generate the store path for the NixOS system
-toplevel:
+Generate the store path that includes all the software and configurations for
+the NixOS system:
 
 ```bash
-nix-build -I nixos-config=/etc/nixos/configuration.nix -E '(import <nixpkgs/nixos> {}).config.system.build.toplevel'
+nixos=$(nix-build configuration.nix -A machine.config.system.build.toplevel' --no-out-path)
 ```
 
-This will output a path in `/nix/store` that corresponds to the system toplevel,
-which includes all the software and configurations for the system. Keep this
-path handy as well.
-
-## Running NixOS-Anywhere
-
-With both paths in hand, you can execute `nixos-anywhere` as follows:
+Run `nixos-anywhere` as follows:
 
 ```bash
-nixos-anywhere --store-paths /nix/store/[your-disk-image-path] /nix/store/[your-toplevel-path]
+nixos-anywhere --store-paths $disko $nixos root@machine
 ```
-
-Replace `[your-disk-image-path]` and `[your-toplevel-path]` with the
-corresponding store paths you generated earlier.