Set RUSTFLAGS in CI to deny warnings

.. this will keep us from re-introducing any new warnings and force us
to handle them.

.github/workflows/build.yml

@@ -42,6 +42,9 @@ jobs:
       - name: Check formatting
         if: matrix.check-fmt
         run: rustup component add rustfmt && cargo fmt --all -- --check
+      - name: Set RUSTFLAGS to deny warnings
+        if: "matrix.toolchain == 'stable'"
+        run: echo "RUSTFLAGS=-D warnings" >> "$GITHUB_ENV"
       - name: Test on Rust ${{ matrix.toolchain }}
         run: |
           cargo test