ci: let cargo check fail on warnings

The rustflags will only affect build process which means that it only
affects cargo check.

Author: fewerner

.github/workflows/rust.yml

@@ -46,7 +46,7 @@ jobs:
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           components: rustfmt, clippy
-          rustflags: ''
+          rustflags: '-D warnings'
           target: wasm32-unknown-unknown
       - run: cargo fmt --all -- --check
       - run: cargo clippy --locked -- -D warnings