Fix build against Nix master

Dependencies

• [2] STZE4KKR Fix build against Nix master
• [3] 4N5APGRG * Start of a helper tool to evaluate job expressions efficiently.
• [4] FUB37KFU Use Nix's restricted evaluation mode
• [5] VTMBJVIS hydra-eval-jobs: Disable the build hook.

Change contents

• replacement in src/hydra-eval-jobs/hydra-eval-jobs.cc at line 198
  [3.105]→[2.755:793](∅→∅)
  − settings.restrictEval = true;
  [3.105]

  [3.854]
  + evalSettings.restrictEval = true;