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.
The fmt test will cause clippy dev to be compiled and run. The clippy dev
dependencies are currently stored at `target/debug/deps` when this happens.
This location sometimes causes conflicts with ui tests which result in the
build error "thread 'compile_test' panicked at 'Found multiple rlibs for crate
`regex` ...".
This commit forces the clippy_dev dependencies to be stored under
`clippy_dev/target/` which seems to resolve this problem.
If you've been using `./util/dev` before, this now becomes `cargo dev`.
The key part of this change is found in `.cargo/config`.
This means one less shell script and a bit more cross-platform support
for contributors.
