stylix: improve stylix.image type (#1414)

Previously:

- Out-of-store paths were not necessarily copied to store.

- Derivations were needlessly coerced to strings.

- `package` and `path` were both used, despite `path` being a superset
  of `package`.

Link: https://github.com/nix-community/stylix/pull/1414

Reviewed-by: awwpotato <awwpotato@voidq.com>
Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>

stylix/palette.nix

@@ -29,7 +29,10 @@ in
     };
 
     image = lib.mkOption {
-      type = with lib.types; nullOr (coercedTo package toString path);
+      type = lib.types.nullOr lib.types.path;
+      # Ensure the path is copied to the store
+      apply =
+        value: if value == null || lib.isDerivation value then value else "${value}";
       description = ''
         Wallpaper image.