config: get rid of impure eval

nixosModules/clanCore/options.nix

@@ -0,0 +1,12 @@
+{ pkgs, options, lib, ... }: {
+  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`
+    '';
+  };
+}