[ci] Configure clippy to fail on warnings

[bkontur]

Before this PR: clippy has one warning, but did not fail:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4672812

warning: field `cache_dir` is never read
  --> polkadot/node/core/pvf/benches/host_prepare_rococo_runtime.rs:32:2
   |
30 | struct TestHost {
   |        -------- field in this struct
31 |     // Keep a reference to the tempdir as it gets deleted on drop.
32 |     cache_dir: tempfile::TempDir,
   |     ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default
warning: `polkadot-node-core-pvf` (bench "host_prepare_rococo_runtime") generated 1 warning

After fix in this PR: clippy fails on warnings.
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4672949

[liamaharon]

Can we be more specific here?

[bkontur]

We could/should, but I don't quite understand the full comment here: https://github.com/paritytech/polkadot-sdk/blob/master/.cargo/config.toml#L7-L12

# An auto defined `clippy` feature was introduced,
# but it was found to clash with user defined features,
# so was renamed to `cargo-clippy`.
#
# If you want standard clippy run:
# RUSTFLAGS= cargo clippy

Maybe we should also change/align that?

@liamaharon @bkchr @ggwpez @alvicsam any suggestions?

[liamaharon]

also pinging @athei and @gilescope in case they remember https://github.com/paritytech/substrate/pull/9694/files#r716463069

[gilescope]

If we've updated the compiler to 1.74 then there's a clippy PR that we can land that removes our usage of the hack in .cargo/config.toml as we can now store lints in the workspace toml file. I think in my PR there's a couple of new crates that need to have their Cargo.tomls tweaked and then it should run green. #2390

[bkontur]

@gilescope
Giles, cool, thank you for the comment :)
Do you plan to finish #2390? If not, I could. Just, please, let me know.

CI has 1.74:

$ cargo --version
cargo 1.74.0 (ecb9851af 2023-10-18)
...
$ cargo +nightly --version
cargo 1.75.0-nightly (b4d18d4bd 2023-10-31)

[bkontur]

closing this in favor of #2390 which is fixed now