yeah single threaded builds seem more reliable but also quite a price to pay (https://github.com/NixOS/nixpkgs/issues/151347#issuecomment-997817696, https://gitlab.haskell.org/ghc/ghc/-/issues/12935#note_373865)
perhaps an option for some small/fast modules that are currently unreproducible, but it might be good to put some energy in looking into the upstream ghc problem(s?) as well