chore(ci): print git diff on librarian check failure (#13450)

Update the `Check for generated code changes` step in
the`librarian_generation_check.yaml` workflow to print the `git diff` to
the console when changes are detected. This will help identify which
files were modified and what the changes are directly from the CI logs.

Fixes googleapis/librarian#6406

Author: zhumin8

.github/workflows/librarian_generation_check.yaml

@@ -73,6 +73,9 @@ jobs:
       run: |
         if [ -n "$(git status --porcelain)" ]; then
           git status
+          echo "==================== GIT DIFF ===================="
+          git diff
+          echo "=================================================="
           echo "Regeneration produced code changes! Please run 'librarian generate --all' to update the generated files."
           exit 1
         fi