Set proper charset on log files

Dependencies

• [2] FHVJYJFE Upload build logs to the binary cache
• [3] UVQJBDHN Move log compression to a plugin
• [*] MHVIT4JY Split hydra-queue-runner.cc more

Change contents

• replacement in src/hydra-queue-runner/builder.cc at line 157
  [2.335]→[2.335:449](∅→∅)
  − store->upsertFile("log/" + baseNameOf(buildDrvPath), readFile(result.logFile), "text/plain");
  [2.335]

  [2.449]
  + store->upsertFile("log/" + baseNameOf(buildDrvPath), readFile(result.logFile), "text/plain; charset=utf-8");