ci/update: use if then else for setting cancelled step output

The simpler `[[ ]] &&` construct can exit non-zero, causing the workflow
to fail.

.github/workflows/update.yml

@@ -129,9 +129,15 @@ jobs:
           (
             echo "old_rev=$old"
             echo "new_rev=$new"
-            [[ "$old" = "$new" ]] && echo 'cancelled=1'
           ) >> $GITHUB_OUTPUT
 
+          if [[ "$old" = "$new" ]]; then
+            echo "Old and new revisions are the same"
+            echo 'cancelled=1' >> $GITHUB_OUTPUT
+          else
+            echo "Old and new revisions are different"
+          fi
+
       - name: Update generated files
         id: generate
         if: (!steps.changes.outputs.cancelled) && (inputs.generate || github.event_name == 'schedule')