replace #[cfg_attr(rustfmt, rustfmt_skip)] with #[rustfmt::skip]

[gnzlbg]

It would be nice to have a clippy lint for porting the old rustfmt_skip attribute to the new rustfmt::skip one.

[flip1995]

I worked on a lint for cfg_attr(clippy) before, but failed at inner attributes:
https://github.com/flip1995/rust-clippy/commits/cfg_attr_lint

But since inner attributes aren't really relevant for cfg_attr(rustfmt) this code can be used pretty much as it is right now for rustfmt. If someone is interested in implementing this, feel free to reuse this code!