Use rustup provided clippy.

[Hoverbear]

Also see: https://internals.rust-lang.org/t/clippy-is-available-as-a-rustup-component/7967

Move the clippy we use from being installed by cargo to being installed by rustup. If you find strange problems with clippy after they are merged you will need to do: cargo uninstall clippy && cargo uninstall clippy-lints && rustup self update

This is because the old versions of rustup don't know that clippy is managed, so they give errors like subcommand clippy does not exist.

Reasons for doing this:

Currently the clippy version must be managed separate from the Rust nightly. Using Rustup will allow each of our projects to depend on their own clippy versions and help avoid some toolchain related problems.

The PRs:

• https://github.com/pingcap/raft-rs/pull/95
• https://github.com/pingcap/rust-prometheus/pull/186
• https://github.com/pingcap/grpc-rs/pull/208
• https://github.com/pingcap/fail-rs/pull/13
• https://github.com/pingcap/tikv/pull/3335

[siddontang]

PTAL @overvenus

[siddontang]

@overvenus please follow the rule to give LGTM or approve it