[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 <>

.gitlab/pipeline/test.yml

@@ -438,6 +438,7 @@ cargo-check-each-crate:
     - .run-immediately
     # - .collect-artifacts
   variables:
+    RUSTFLAGS: "-D warnings"
     # $CI_JOB_NAME is set manually so that cache could be shared for all jobs
     # "cargo-check-each-crate I/N" jobs
     CI_JOB_NAME: cargo-check-each-crate