add disko to all installer

Since disko is just some shell scripts it's comparable small and we can
easily embed this everywhere.
We can make use of this in our NixOS tutorials by providing nicer
images.

nix/installer.nix

@@ -36,6 +36,8 @@
     # alternative to nixos-generate-config
     # TODO: use nixpkgs again after next nixos release
     (pkgs.callPackage ./nixos-facter.nix {})
+
+    pkgs.disko
   ];
 
   # Don't add nixpkgs to the image to save space, for our intended use case we don't need it