Fix not-null constraint violation inserting build step

Dependencies

• [2] SX5XYD6I When propagating failure, propagate the duration and machine
• [3] JGLE5BRN Add separate build step status codes for cached failures and timeouts
• [4] LZO3C2KI * Hack around those SQLite timeouts: just retry the transaction.
• [*] 7YBYT2LQ

Change contents

• replacement in src/script/hydra-build at line 213
  [2.328]→[2.328:365](∅→∅)
  − my $machine = undef;
  [2.328]

  [3.1010]
  + my $machine = "";