make-squashfs: drop references to build closure

nixos/lib/make-squashfs.nix

@@ -27,6 +27,10 @@ stdenv.mkDerivation {
   name = "${fileName}${lib.optionalString (!hydraBuildProduct) ".img"}";
   __structuredAttrs = true;
 
+  # the image will be self-contained so we can drop references
+  # to the closure that was used to build it
+  unsafeDiscardReferences.out = true;
+
   nativeBuildInputs = [ squashfsTools ];
 
   buildCommand =