make lanzaboote.nix use lib.getExe for lzbt bin

nix/modules/lanzaboote.nix

@@ -131,7 +131,7 @@ in
 
         # Use the system from the kernel's hostPlatform because this should
         # always, even in the cross compilation case, be the right system.
-        ${cfg.package}/bin/lzbt install \
+        ${lib.getExe cfg.package} install \
           --system ${config.boot.kernelPackages.stdenv.hostPlatform.system} \
           --systemd ${config.systemd.package} \
           --systemd-boot-loader-config ${loaderConfigFile} \