ci: deny warnings

Author: Jonas Schievink

.github/workflows/ci.yml

@@ -34,7 +34,12 @@ jobs:
         thumbv8m.main-none-eabi
     - name: Build Crates
       run: cargo test -p xtask
+      env:
+        RUSTFLAGS: ${{ matrix.rustflags }}
 
     strategy:
       matrix:
-        rust_version: [stable, beta, nightly]
+        rust_version: [beta, nightly]
+        include:
+          - rust_version: stable
+            rustflags: --deny warnings