Rustfix does not work when using a workspace

[Lapz]

When using rustfix in a workspaced project it fails to apply the correct fixes and gives an IO error.

[killercup]

Rustup is not currently aware of workspaces at all. We should make sure it does something useful, and then probably also add some features to let users decide which workspace member to run it on.

[alexcrichton]

I think this was fixed in #66, so closing