add clippy to CI (#9694)

* Focus on correctness lints. This relies on a compiler patch that landed 8th July 2021.
If people are using an earlier version of the compiler everything will still work unless they try and run clippy.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Denis Pisarev <denis.pisarev@parity.io>

substrate/.gitlab-ci.yml

@@ -311,7 +311,7 @@ cargo-deny:
     when:                          always
     paths:
       - deny.log
-  # FIXME: Temorarily allow to fail.
+  # FIXME: Temporarily allow to fail.
   allow_failure:                   true
 
 cargo-fmt:
@@ -321,6 +321,13 @@ cargo-fmt:
   script:
     - cargo +nightly fmt --all -- --check
 
+cargo-clippy:
+  stage:                           test
+  <<:                              *docker-env
+  <<:                              *test-refs
+  script:
+    - SKIP_WASM_BUILD=1 env -u RUSTFLAGS cargo +nightly clippy
+
 cargo-check-benches:
   stage:                           test
   <<:                              *docker-env