Remove gc-check-reachability setting

Dependencies

• [2] D3MDJONY
• [3] 3FTM52B2 hydra-module: Always GC to have at least 100 GiB available.
• [4] 5DEWIOO4 hydra-module.nix: Export the HYDRA_* environment variables
• [5] RNY3XV2G Remove Nix options not needed by the new queue runner

Change contents

• edit in hydra-module.nix at line 246
  [2.3321]→[2.3321:3459](∅→∅)
  −

  − # The default (`true') slows Nix down a lot since the build farm

  − # has so many GC roots.

  − gc-check-reachability = false