config: get rid of impure eval

pkgs/clan-cli/tests/test_flake/flake.nix

@@ -9,12 +9,24 @@
         (if builtins.pathExists ./machines/machine1/settings.json
         then builtins.fromJSON (builtins.readFile ./machines/machine1/settings.json)
         else { })
-        {
-          nixpkgs.hostPlatform = "x86_64-linux";
-          # speed up by not instantiating nixpkgs twice and disable documentation
-          nixpkgs.pkgs = inputs.nixpkgs.legacyPackages.x86_64-linux;
-          documentation.enable = false;
-        }
+        ({ lib, options, pkgs, ... }: {
+          config = {
+            nixpkgs.hostPlatform = "x86_64-linux";
+            # speed up by not instantiating nixpkgs twice and disable documentation
+            nixpkgs.pkgs = inputs.nixpkgs.legacyPackages.x86_64-linux;
+            documentation.enable = false;
+          };
+          options.clanCore.optionsNix = lib.mkOption {
+            type = lib.types.raw;
+            internal = true;
+            readOnly = true;
+            default = (pkgs.nixosOptionsDoc { inherit options; }).optionsNix;
+            defaultText = "optionsNix";
+            description = ''
+              This is to export nixos options used for `clan config`
+            '';
+          };
+        })
       ];
     };
   };