custom_target will substitute @PRIVATE_DIR@ for the patch to a directory for scratch data. It would be nice to have something similar for tests. Or is there another portable way to get a temporary directory?

Asking on behalf of this comment https://github.com/NixOS/nix/pull/13370/files/02e5588bf9e8b9288f0c5e1ad74d88574b9a65b7#r2151763078