.github: cargo clippy with --deny warnings option

To make the CI correctly work when running cargo clippy, an additional
option `-- --deny warnings` is needed. Without that, cargo clippy would
always succeed, so CI is not able to check its result.

.github/workflows/ci.yaml

@@ -38,4 +38,4 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: clippy
-          args: --workspace
+          args: --workspace -- --deny warnings