hydra-queue-runner: Handle restarted builds whose derivation is gone

Dependencies

• [2] TPSCSZKX Speed up findBuildDependencyInQueue
• [*] 7YBYT2LQ
• [*] YEXD7CBK Fix findBuildDependencyInQueue

Change contents

• edit in src/script/hydra-queue-runner at line 51
  [2.36]

  [5.116]
  + return undef unless isValidPath($build->drvpath);