Handle derivations without a system attribute

Dependencies

• [2] POPU2ATH * hydra_scheduler: use eval-jobs.
• [*] 4N5APGRG * Start of a helper tool to evaluate job expressions efficiently.
• [*] 4LWGZL33

Change contents

• edit in src/c/hydra-eval-jobs.cc at line 131
  [5.748]

  [2.6]
  + if (drv.system == "unknown")

  + throw EvalError("derivation must have a ‘system’ attribute");

  +