t/Setup.pm: sort NIX_ env vars

Dependencies

• [2] 4WLW4VHS Test setup: support arbitrary hydra config
• [3] OWRS526H Create an ephemeral PostgreSQL database per test
• [4] UIA3ULNU Give each test its own Nix directories
• [*] HX4QYOYA add first evaluations tests

Change contents

• edit in t/lib/Setup.pm at line 58
  [2.1085]

  [3.1894]
  + $ENV{'NIX_LOG_DIR'} = "$dir/nix/var/log/nix";
• edit in t/lib/Setup.pm at line 61
  [3.2051]→[3.2051:2101](∅→∅)
  − $ENV{'NIX_LOG_DIR'} = "$dir/nix/var/log/nix";