Store `environment-markers` in solve order (#6078)
## Summary
Right now, we store the environment markers in a `BTreeSet` -- so
they're sorted, but the sort doesn't really tell us anything. I think we
should instead store them in the order in which we solved. I thought
this might fix an instability (it didn't), but I think it's still good
to ensure we solve in the same order.
I also changed from `Option<Vec>` to just `Vec`, since there was no
distinction between `None` and empty.
