t/lib/Setup.pm: nix_config: default to empty string

Dependencies

• [2] HXEZUCTD Add test to verify includes work
• [3] UIA3ULNU Give each test its own Nix directories
• [4] 4WLW4VHS Test setup: support arbitrary hydra config
• [*] HX4QYOYA add first evaluations tests

Change contents

• replacement in t/lib/Setup.pm at line 53
  [3.1767]→[2.576:640](∅→∅)
  − my $nix_config = "sandbox = false\n" . $opts{'nix_config'};
  [3.1767]

  [2.640]
  + my $nix_config = "sandbox = false\n" . ($opts{'nix_config'} || "");