CI: deny warnings.

.github/workflows/rapier-ci-build.yml

@@ -18,6 +18,8 @@ jobs:
       run: cargo fmt -- --check
   build-native:
     runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -D warnings
     steps:
     - uses: actions/checkout@v2
     - run: sudo apt-get install -y cmake libxcb-composite0-dev
@@ -49,6 +51,8 @@ jobs:
       run: cargo check -j 1 --verbose -p rapier-examples-3d;
   build-wasm:
     runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -D warnings
     steps:
       - uses: actions/checkout@v2
       - run: rustup target add wasm32-unknown-unknown