Auto merge of #5363 - yaahc:clippy-fix, r=phansch,flip1995
Auto merge of #5363 - yaahc:clippy-fix, r=phansch,flip1995

add --fix support to `cargo-clippy`

Prior to this we had started work on integrating clippy as a subcommand directly into cargo in the form of `cargo clippy-preview` and `cargo fix --clippy`. In the course of that work it was decided that the best approach would be to strictly add the features clippy needed to cargo in order to insert `clippy-driver` only for workspace crates. This was accomplished by adding a `RUSTC_WORKSPACE_WRAPPER` env variable to cargo that will override the normal `RUSTC_WRAPPER` when both are present and the current crate is a workspace crate.

This change adds support to clippy to use this by setting the `RUSTC_WORKSPACE_WRAPPER` env variable instead `RUSTC_WRAPPER` and by detecting `--fix` as an arg and swapping out the `check` cargo command for `fix` when it is present.

WIP, here are the current issues that I still need to resolve

- [x] Detect if we're running on nightly rust
  - [x] Set `RUSTC_WORKSPACE_WRAPPER` on nightly, and `RUSTC_WRAPPER` on stable
  - [x] Error out on stable when `--fix` is specified, because stable currently hasn't landed the PR for `RUSTC_WORKSPACE_WRAPPER` so if we set this it just runs check and silently fails
- [ ] Update the help text
  - [ ] The current plan is to shell out to `cargo check --help` and then postprocess the output to mention clippy instead of check where appropriate and to add the extra info about `--fix` and the `-- -A lint` options.
- [x] tests?

changelog: add `--fix` arg to `cargo-clippy`

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust