wfvm/demo-image: make buildable from flake

flake.nix

@@ -16,5 +16,9 @@
       lib = import ./wfvm {
         inherit pkgs;
       };
+
+      packages.${system}.demoImage = import ./wfvm/demo-image.nix {
+        inherit self;
+      };
     };
 }

wfvm/demo-image.nix

@@ -1,7 +1,17 @@
-{ pkgs ? import <nixpkgs> {}, impureMode ? false }:
+{ pkgs ? import <nixpkgs> {}
+# Whether to generate just a script to start and debug the windows installation
+, impureMode ? false
+# Flake input `self`
+, self ? null
+}:
 
 let
-  wfvm = (import ./default.nix { inherit pkgs; });
+  wfvm =
+    if self == null
+    # nix-build
+    then (import ./default.nix { inherit pkgs; })
+    # built from flake.nix
+    else self.lib;
 in
 wfvm.makeWindowsImage {
   # Build install script & skip building iso