Make cargo doc fail on warnings in the CI

Author: fvictorio

.github/workflows/hardhat-core-ci.yml

@@ -295,6 +295,8 @@ jobs:
 
       - name: Cargo doc
         uses: actions-rs/cargo@v1
+        env:
+          RUSTDOCFLAGS: "-D warnings"
         with:
           command: doc
           args: --workspace --no-deps --all-features