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/client/executor/src/wasm_runtime.rs

@@ -304,7 +304,7 @@ pub fn create_wasm_runtime_with_code(
 			//
 			// We drop the cache_path here to silence warnings that cache_path is not used if
 			// compiling without the `wasmtime` flag.
-			drop(cache_path);
+			let _ = cache_path;
 
 			sc_executor_wasmi::create_runtime(
 				blob,