Auto merge of #7956 - ehuss:fix-collision-test, r=alexcrichton

Auto merge of #7956 - ehuss:fix-collision-test, r=alexcrichton

Fix rare failure in collision_export test.

Seen once on CI in #7952 (https://dev.azure.com/rust-lang/cargo/_build/results?buildId=22112&view=logs&j=a5e52b91-c83f-5429-4a68-c246fc63a4f7&t=d4864165-4be3-5e34-b483-a6b05303aa68). I was able to reproduce it locally, though it is rare. There seems to be some kind of race issue on macOS with two processes trying to symlink the same directory at the same time. The solution is to serialize the build so they don't run at the same time.