--end flag

[BoySanic]

We have a --start flag to specify which seed to start from, but I'd like an --end flag we can use to specify an end seed as well.

It doesn't have to completely cease execution at the exact seed specified in --end. It just needs to exit the infinite loop once start_seed would exceed --end.

In other words, it can run seeds beyond --end so long as it does so deterministically. Like maybe --end occurs in the middle of a batch of seeds, so it's convenient to let it run the full batch then exit instead of finding some janky way to exit mid-batch.