[ci] Add -D warnings for cargo-check-each-crate job to fail on warnings (#2670)

## Summary

This PR turns on `-D warnings` for `cargo-check-each-crate job` job to
fail on warnings e.g. like this:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673130

Before this PR, there was a warning and `cargo-check-each-crate` job did
not fail:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4641444
```
warning: unused import: `ToTokens`
  --> substrate/primitives/api/proc-macro/src/utils.rs:22:34
   |
22 | use quote::{format_ident, quote, ToTokens};
   |                                  ^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default
warning: `sp-api-proc-macro` (lib) generated 1 warning (run `cargo fix --lib -p sp-api-proc-macro` to apply 1 suggestion)
```

Fixes on the way:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4641444
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673265
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673410
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673681
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673836
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4673941
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4674256
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4679328



## Questions
- [ ] why does this check triggers only `cargo check --locked`?
`--all-features` or `--all-targets` are not needed? Or aren't they
avoided intentionally?

---------

Co-authored-by: command-bot <>

substrate/frame/contracts/proc-macro/src/lib.rs

@@ -74,7 +74,7 @@ fn derive_debug(input: TokenStream, fmt: impl Fn(&Ident) -> TokenStream2) -> Tok
 	#[cfg(not(feature = "full"))]
 	let fields = {
 		drop(fmt);
-		drop(data);
+		let _ = data;
 		TokenStream2::new()
 	};