nixos/maintainers/scripts/lxd: simplify config generation

Use modulesPath so we don't have to magically rewrite paths in activation script,
set stateVersion to the one this was built with (which should approximate "first install")

nixos/maintainers/scripts/lxd/lxd-container-image-inner.nix

···
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, modulesPath, ... }:
 
 {
   imports =
     [
       # Include the default lxd configuration.
-      ../../../modules/virtualisation/lxc-container.nix
+      "${modulesPath}/modules/virtualisation/lxc-container.nix"
       # Include the container-specific autogenerated configuration.
       ./lxd.nix
     ];
···
   networking.useDHCP = false;
   networking.interfaces.eth0.useDHCP = true;
 
-  system.stateVersion = "21.05"; # Did you read the comment?
+  system.stateVersion = "@stateVersion@"; # Did you read the comment?
 }

nixos/maintainers/scripts/lxd/lxd-container-image.nix

···
   };
 
   # copy the config for nixos-rebuild
-  system.activationScripts.config = ''
+  system.activationScripts.config = let
+    config = pkgs.substituteAll {
+      src = ./lxd-container-image-inner.nix;
+      stateVersion = lib.trivial.release;
+    };
+  in ''
     if [ ! -e /etc/nixos/configuration.nix ]; then
       mkdir -p /etc/nixos
-      cat ${./lxd-container-image-inner.nix} > /etc/nixos/configuration.nix
-      ${lib.getExe pkgs.gnused} 's|../../../modules/virtualisation/lxc-container.nix|<nixpkgs/nixos/modules/virtualisation/lxc-container.nix>|g' -i /etc/nixos/configuration.nix
+      cp ${config} /etc/nixos/configuration.nix
     fi
   '';
 

nixos/maintainers/scripts/lxd/lxd-virtual-machine-image-inner.nix

···
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, modulesPath, ... }:
 
 {
   imports =
     [
       # Include the default lxd configuration.
-      ../../../modules/virtualisation/lxd-virtual-machine.nix
+      "${modulesPath}/virtualisation/lxd-virtual-machine.nix"
       # Include the container-specific autogenerated configuration.
       ./lxd.nix
     ];
···
   networking.useDHCP = false;
   networking.interfaces.eth0.useDHCP = true;
 
-  system.stateVersion = "23.05"; # Did you read the comment?
+  system.stateVersion = "@stateVersion@"; # Did you read the comment?
 }

nixos/maintainers/scripts/lxd/lxd-virtual-machine-image.nix

···
   };
 
   # copy the config for nixos-rebuild
-  system.activationScripts.config = ''
+  system.activationScripts.config = let
+    config = pkgs.substituteAll {
+      src = ./lxd-virtual-machine-image-inner.nix;
+      stateVersion = lib.trivial.release;
+    };
+  in ''
     if [ ! -e /etc/nixos/configuration.nix ]; then
       mkdir -p /etc/nixos
-      cat ${./lxd-virtual-machine-image-inner.nix} > /etc/nixos/configuration.nix
-      ${lib.getExe pkgs.gnused} 's|../../../modules/virtualisation/lxd-virtual-machine.nix|<nixpkgs/nixos/modules/virtualisation/lxd-virtual-machine.nix>|g' -i /etc/nixos/configuration.nix
+      cp ${config} /etc/nixos/configuration.nix
     fi
   '';