chore(3p/nixpkgs): bump channels (2025-07-31)

Change-Id: I1ae84e743281125ecbdc98cdb4f2adf5a96905a7
Reviewed-on: https://cl.snix.dev/c/snix/+/30628
Reviewed-by: Ryan Lahfa <ryan@lahfa.xyz>
Autosubmit: Florian Klink <flokli@flokli.de>
Tested-by: besadii

third_party/nixpkgs/default.nix

@@ -33,6 +33,10 @@ let
         # Forbids our meta.ci attribute
         # https://github.com/NixOS/nixpkgs/pull/191171#issuecomment-1260650771
         checkMeta = false;
+        permittedInsecurePackages = [
+          # only used inside tests
+          "nix-2.3.18"
+        ];
       };
 
     inherit localSystem crossSystem;