Otherwise tests may fail with wide character errors.
In https://github.com/NixOS/nixpkgs/pull/57677 default postgresql socket directory was changed to `/run/postgresql`, which doesn't exist (and can't be created) in Nix build environment. We'll use /tmp as socket dir explicitly then. Fixes build failure https://hydra.nixos.org/build/91221682 Cc @aszlig @edolstra
Also, if the machines file contains an entry for localhost, then run "nix-store --serve" directly, without going through SSH.