Fix nix CI: add diffutils parameter to agda2hs.nix

The testsuite uses diff, so diffutils needs to be available.
In agda2hs.nix, pkgs refers to the list of Agda packages (not nixpkgs),
so diffutils must be passed as a function parameter via callPackage.

Co-authored-by: andreasabel <[email protected]>

Author: Copilot

nix/agda2hs.nix

@@ -12,6 +12,7 @@
   makeWrapper,
   writeText,
   ghcWithPackages,
+  diffutils,
 }:
 
 let
@@ -52,7 +53,7 @@ let
       {
         inherit pname version;
         nativeBuildInputs = [
-          pkgs.diffutils
+          diffutils
           makeWrapper
         ];
         passthru = {