running cargo in a custom toolchain is racy

[rbtcollins]

This is probably best tackled after #988 is fixed. Running cargo in a custom toolchain unlinks and copies in a copy of cargo into a common well known fallback dir. This can race if run concurrently, leading to spurious errors.

There are a couple possible errors: failure to unlink due to cargo still running; failure to execute due to cargo unlinked by another process.