Fix registerRoot

Dependencies

• [2] 4LL3WLON Write Hydra roots as regular files instead of symlinks
• [*] 2GK5DOU7 * Downloading closures.
• [*] A63IHCMX * Register GC roots properly.

Change contents

• edit in src/lib/Hydra/Helper/Nix.pm at line 82
  [5.968]

  [2.0]
  + return if -e $link;