ci: Fail build on warnings

Author: projectgus

.github/workflows/ci.yml

@@ -51,6 +51,8 @@ jobs:
           key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Build
+        env:
+          RUSTFLAGS: '-D warnings'
         run: cargo build --all --verbose
       - name: Tests
         run: cargo test --all --verbose