github-ci: deny warnings in clippy check

We currently have some clippy warnings in the ffi crate. They output to
to the terminal but clippy exits with a success code. Typically we want
to fix this, so deny warnings which will cause clippy to exit with an
error code, failing ci.

.github/workflows/builds.yml

@@ -366,7 +366,7 @@ jobs:
               echo "::error ::Clippy --fix made changes, please fix"
               exit 1
           fi
-      - run: cargo clippy --all-features --all-targets
+      - run: cargo clippy --workspace --all-features --all-targets -- -D warnings
         working-directory: rust
       # especially without debug feature
       - run: cargo clippy