output git diff in CI to debug when a diff check fails

.github/workflows/tests.yml

@@ -35,6 +35,7 @@ jobs:
               # skip this check in windows for now, as the build outputs may get slightly modified in Windows, which we want to fix.
               if: runner.os != 'Windows'
               run: |
+                  git diff
                   git add . && git diff --quiet && git diff --cached --quiet
               env:
                   CI: true