tests: Make the external destination store optional

Dependencies

• [2] C66VQYWT Run the tests with a remote dest store
• [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 57
  [3.1026]→[2.0:56](∅→∅)
  − print $fh "store_uri = file:$dir/nix/dest-store\n";
  [3.1026]

  [3.1026]
  + if ($opts{'use_external_destination_store'} // 1) {

  + print $fh "store_uri = file:$dir/nix/dest-store\n"

  + }