Auto merge of #4724 - pornel:sortpaths, r=alexcrichton

Sort native library paths for deterministic builds

Fixes #3800 by using a `BTreeSet`, which guarantees a deterministic iteration order.

Since the order was previously arbitrary, a sorted order is just as good. The list is so small, that any performance difference between `BTreeSet` and `HashSet` is negligible.

diff --cc src/cargo/ops/cargo_rustc/compilation.rs
Simple merge