Rollup merge of #121137 - GuillaumeGomez:add-clippy-cfg, r=Urgau,Nilstrieb

Add clippy into the known `cfg` list

In clippy, we are removing the `feature = "cargo-clippy"` cfg to replace it with `clippy` in https://github.com/rust-lang/rust-clippy/pull/12292. But for it to work, we need to declare `clippy` as cfg. It makes it more coherent with other existing tools like rustdoc.

cc `@flip1995`