update source attributes other than `src`

[Artturin]

Motivation is to update packages such as catppuccin CC @khaneliman and lambda-lisp CC @cafkafk

Diff Setting bat source to an old version and adding sources to passthru

diff --git a/pkgs/data/themes/catppuccin/default.nix b/pkgs/data/themes/catppuccin/default.nix
index 16befcd06b81..46214c6ed74b 100644
--- a/pkgs/data/themes/catppuccin/default.nix
+++ b/pkgs/data/themes/catppuccin/default.nix
@@ -20,8 +20,8 @@ let
       name = "bat";
       owner = "catppuccin";
       repo = "bat";
-      rev = "ba4d16880d63e656acced2b7d4e034e4a93f74b1";
-      hash = "sha256-6WVKQErGdaqb++oaXnY3i6/GuH2FhTgK0v4TN4Y0Wbw=";
+      rev = "00bd462e8fab5f74490335dcf881ebe7784d23fa";
+      hash = "sha256-yzn+1IXxQaKcCK7fBdjtVohns0kbN+gcqbWVE4Bx7G8=";
     };
 
     bottom = fetchFromGitHub {
@@ -102,6 +102,8 @@ stdenvNoCC.mkDerivation {
     runHook postInstall
   '';
 
+  passthru = { inherit sources; };
+
   meta = {
     description = "Soothing pastel themes";
     homepage = "https://github.com/catppuccin/catppuccin";

nix-update "catppuccin" --src-attr "sources.bat"

It would be nice to be able to use nix-update to update such sources instead of a custom update script

Related https://github.com/Mic92/nix-update/issues/163 because being able to pass catppuccin.sources.bat would fix this issue too.