rustfmt is not available in latest nightly

```
error: component 'rustfmt' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel 'nightly'
```

https://github.com/rust-lang/rustup.rs/issues/2047

See also:
https://github.com/rust-lang/rustup.rs/issues/1566
https://github.com/rust-lang/rustup.rs/issues/1958