Fixes#8248
According to https://doc.rust-lang.org/cargo/reference/unstable this
seems to be the right place to set it, and it does fix the build for me.
I haven't removed the other `rustflags` because perhaps it's needed on
different cargo/rust versions?
`.cargo/config.toml` is the preferred form for the local configuration
file. This is emphasized in _The Cargo Book_ with the following note:
> Note: Cargo also reads config files without the `.toml` extension, such
> as `.cargo/config`. Support for the `.toml` extension was added in version
> 1.39 and is the preferred form.
Moreover, this helps with toml-aware text editors as they will recognize
the file extension.
Alex Macleod