Fix invalid generated dn42 IPv6 for pulsebert

Before the C3D2 prefix ended in :: and the final address contained :::
which is invalid.

config/c3d2.nix

@@ -3,9 +3,9 @@
 { zentralwerk, hostRegistry, config, options, lib, pkgs, ... }:
 
 let
-  hqPrefix64 = builtins.head (
+  hqPrefix64 = lib.removeSuffix "::" (builtins.head (
     builtins.split "/" zentralwerk.lib.config.site.net.c3d2.subnets6.dn42
-  );
+  ));
 
   # Generate a deterministic IPv6 address for a 64 bit prefix
   # and seed string. Prefix must not contain trailing ':'.