rustup should not delete the entire ~/.rustup/tmp dir

[tesuji]

Rustup should only remove the tmp/<toolchain> dir that it downloaded

When rustup-toolchain-install-master (RTIM) and rustup update run concurrently, rustup often finishs
installation first (probably because of nightly toolchain is cache by cloudfont, I don't know), so
the tmp is removed by rustup. This cause RTIM fails to install the master toolchain.

Describe the solution you'd like

I don't have a clear suggestion. But I think rustup could keep the tmp dir, and only remove
the toolchain that it is going to install.

Notes

[kinnison]

At this time rustup does not support being run in parallel for any activity which could use or alter toolchains. I believe @rbtcollins is considering locking approaches to ameliorate this kind of issue.

The post full rustup update temp dir cleanup is explicitly there as a way to resolve issues with partially downloaded content which is not longer referenceable (e.g. cancelled nightly updates)

[rbtcollins]

This is possibly a dupe of #988, possibly related.