ci: add stable cargo fmt check

run clippy only on ubuntu-latest as it doesn't seem to be installed on
windows. however all code is processed by rustfmt regardless of the
target.

.github/workflows/ci.yml

@@ -19,6 +19,8 @@ jobs:
       run: rustup component add clippy
     - if: matrix.rust != 'nightly'
       run: cargo clippy --all-features -- -D warnings
+    - if: matrix.rust == 'stable' && matrix.os == 'ubuntu-latest'
+      run: cargo fmt --all -- --check
     - run: cargo build --no-default-features --verbose
     - run: cargo test --features="master resolv resolv-sync sign tsig validate" --verbose