"cargo miri test" now supports workspace

[RalfJung]

cargo miri test recently was made much more compatible with cargo test, and now supports workspaces in the same way. While checking GitHub for projects that need adjustment to the new way of passing flags to Miri, I stumbled upon this one which does not need updating but has a comment saying "cargo-miri does not support workspaces yet", so I figured I'd leave a note. :)