make-squashfs: use `__structuredAttrs`

Makes it easier to enable discarding of references, a feature of Nix
2.14 which requires structured attrs.

nixos/lib/make-squashfs.nix

···
 
 stdenv.mkDerivation {
   name = "squashfs.img";
+  __structuredAttrs = true;
 
   nativeBuildInputs = [ squashfsTools ];