fix sorting for iterables that define copymutable

[vtjnash]

copymutable is only defined to return an array for abstract arrays, but that is only what this method is never called with. For other types, it has a default of collect (which means the default behavior appears to give an array), but can be changed by other types (such as AbstractSet) to do something different.

Correction to #46104

[LilithHafner]

Since we only support sorting abstract arrays with sort! (and that's all we can support because we need length and random access getindex and setindex) this seems reasonable.

This would be inconvenient for iterable and sortable non-abstractarray types that bring their own copymutable and sort! methods. However, in that case all they have to do is define Base.sort(x::MyType; kw...) = sort!(copymutable(x); kw...), so not a big loss.

collect isn't perrfect; we want collect_to_abstractarray. However, if we ever do add collect_to_abstractarray, I think we could switch this as a minor or non-breaking change.

[LilithHafner]

LGTM, though we should probably wait to see if triage decides to revert #46104 before we merge this.

[vtjnash]

We don't need triage to approve merging a bugfix, even if there may be later changes to the API