bightbox-image.nix: use lib in stead of stdenv.lib. Fixes #18208

nixos/modules/virtualisation/brightbox-image.nix

···
 
           postVM =
             ''
-              PATH=$PATH:${stdenv.lib.makeBinPath [ pkgs.gnutar pkgs.gzip ]}
+              PATH=$PATH:${lib.makeBinPath [ pkgs.gnutar pkgs.gzip ]}
               pushd $out
               ${pkgs.qemu_kvm}/bin/qemu-img convert -c -O qcow2 $diskImageBase nixos.qcow2
               rm $diskImageBase