Fix Clippy lints not causing CI workflow failure

It's easy to miss them out if CI is not aborted on a Clippy lint, and
this behavior change was an unintended regression when CodeQL
integration was added, so let's change it back.

.github/workflows/oxipng.yml

@@ -125,6 +125,7 @@ jobs:
       - name: Run Clippy for all feature combinations
         if: matrix.target == 'x86_64-unknown-linux-gnu'
         run: >
+          set -o pipefail;
           cargo hack clippy --no-deps --all-targets --feature-powerset --exclude-features sanity-checks --message-format=json -- -D warnings
           | clippy-sarif
           | tee clippy-results.sarif