Backport PR matplotlib#18227: Set pipefail when running flake8 linter.

Author: tacaswell

.github/workflows/reviewdog.yml

@@ -28,6 +28,7 @@ jobs:
         env:
           REVIEWDOG_GITHUB_API_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          set -o pipefail
           flake8 --docstring-convention=all | \
             reviewdog -f=pep8 -name=flake8 \
               -tee -reporter=github-check -filter-mode nofilter