fetchzip: simplify postFetch as per suggestions

of @veprbl, @adisbladis, and @MetaDark

pkgs/build-support/fetchzip/default.nix

@@ -47,17 +47,11 @@
     '')
     + ''
       ${extraPostFetch}
-
-      # Remove write permissions for files unpacked with write bits set
-      # Fixes https://github.com/NixOS/nixpkgs/issues/38649
-      #
-      # However, we should (for the moment) retain write permission on the directory
-      # itself, to avoid tickling https://github.com/NixOS/nix/issues/4295 in
-      # single-user Nix installations. This is because in sandbox mode we'll try to
-      # move the path, and if we don't have write permissions on the directory,
-      # then we can't update the ".." entry.
-      chmod -R a-w "$out"
-      chmod u+w "$out"
+    ''
+    # Remove non-owner write permissions
+    # Fixes https://github.com/NixOS/nixpkgs/issues/38649
+    + ''
+      chmod 755 "$out"
     '';
 } // removeAttrs args [ "stripRoot" "extraPostFetch" ])).overrideAttrs (x: {
   # Hackety-hack: we actually need unzip hooks, too