vinyl: use broadcast instead of signal to notify about dump completion

There may be more than one fiber waiting on `vy_scheduler::dump_cond`:

```
box.snapshot
  vinyl_engine_wait_checkpoint
    vy_scheduler_wait_checkpoint

space.create_index
  vinyl_space_build_index
    vy_scheduler_dump
```

To avoid hang, we should use `fiber_cond_broadcast`.

Closes #10233

NO_DOC=bug fix

(cherry picked from commit 30547157)