Switch impure mode to parameter

artiq-fast/windows/README.md

@@ -14,7 +14,7 @@ Results in a file called c.img
 
 If in pure mode
 ```shell
-nix-build
+nix-build build.nix
 ls -la ./result
 ```
 Results in a symlink to the image in the nix store

artiq-fast/windows/build.nix

@@ -1,4 +1,7 @@
-{ pkgs ? import <nixpkgs> {} }:
+{
+  pkgs ? import <nixpkgs> {}
+  , impureMode ? false
+}:
 
 let
   win = (import ./default.nix { inherit pkgs; });
@@ -25,7 +28,7 @@ win.makeWindowsImage {
   };
 
   # Build install script & skip building iso
-  impureMode = false;
+  inherit impureMode;
 
   # impureShellCommands = [
   #   "powershell.exe echo Hello"

artiq-fast/windows/win.nix

@@ -12,9 +12,7 @@
 let
   # qemu_test is a smaller closure only building for a single system arch
   qemu = pkgs.qemu_test;
-  libguestfs = pkgs.libguestfs-with-appliance.override {
-    inherit qemu;
-  };
+  libguestfs = pkgs.libguestfs-with-appliance;
 
   runQemuCommand = name: command: (
     pkgs.runCommandNoCC name { buildInputs = [ pkgs.p7zip qemu libguestfs ]; }