Deny warnings in stable CI

Author: tnull

.github/workflows/build.yml

@@ -50,6 +50,9 @@ jobs:
           cargo update -p zstd-sys --precise "2.0.8+zstd.1.5.5" --verbose # zstd-sys 2.0.9+zstd.1.5.5 requires rustc 1.64.0
           cargo update -p petgraph --precise "0.6.3" --verbose # petgraph v0.6.4, requires rustc 1.64 or newer
           cargo update -p home --precise "0.5.5" --verbose # home v0.5.9, requires rustc 1.70 or newer
+      - name: Set RUSTFLAGS to deny warnings
+        if: "matrix.toolchain == 'stable'"
+        run: echo "RUSTFLAGS=-D warnings" >> "$GITHUB_ENV"
       - name: Build on Rust ${{ matrix.toolchain }}
         run: cargo build --verbose --color always
       - name: Build with UniFFI support on Rust ${{ matrix.toolchain }}