Derivations with multiple outputs break the 'link name is store path' assumption

Dependencies

• [2] UGA45FNC Add a plugin for backing up builds in s3

Change contents

• replacement in src/script/hydra-s3-backup-collect-garbage at line 35
  [2.8512]→[2.8512:8571](∅→∅)
  − push @actual_roots, $Nix::Config::storeDir . "/$link";
  [2.8512]

  [2.8571]
  + push @actual_roots, readlink "$gcRootsDir/$link";