AppVeyor 'stderr: error: unable to unlink old fallback exe' error in dogfood tests

[phansch]

This error started appearing after #3115, but no idea why.

[flip1995]

Especially #3115 (comment)

Probably what happens is... 🤔

We run cargo test and rustup links the fall back file and executes it. During the testing, dogfood calls cargo itself which causes rustup to try and replace the fallback file but it can't because cargo test is still running.

We never had this problem before because we weren't using a custom toolchain.

https://github.com/rust-lang-nursery/rustup.rs/blob/22b6cdb8b000f9217af1dd5dd4e73ce032bdb25d/src/rustup/toolchain.rs#L374

I'm trying to fix this. See rust-lang/rustup#1499.

We fixed this in #3701.