make-disk-image: allow pkgs to be overriden by disko configuration

This is needed when the nixosConfig you're building for modifies nixpkgs options that lead to evaluation failing unnecessarily

lib/make-disk-image.nix

@@ -1,6 +1,6 @@
 { nixosConfig
 , diskoLib
-, pkgs ? nixosConfig.pkgs
+, pkgs ? nixosConfig.config.disko.imageBuilderPkgs
 , lib ? pkgs.lib
 , name ? "${nixosConfig.config.networking.hostName}-disko-images"
 , extraPostVM ? nixosConfig.config.disko.extraPostVM

module.nix

@@ -12,6 +12,15 @@ in
   options.disko = {
     imageBuilderKernel = lib.mkOption {
       type = lib.types.package;
+    imageBuilderPkgs = lib.mkOption {
+      type = lib.types.attrs;
+      description = ''
+        the pkgs instance used when building disk images via make-disk-image.nix.
+        Useful when the config's kernel won't boot in the image-builder.
+      '';
+      default = pkgs;
+      example = lib.literalExpression "pkgs";
+    };
     imageBuilderKernelPackages = lib.mkOption {
       type = lib.types.attrs;
       description = ''