GHA: Batch URIs to invalidate on Akamai

.github/actions/akamai-purge-osx/action.yml

@@ -31,9 +31,11 @@ runs:
           for idx in $indices
           do
             end=${idx%:*}
+            echo "Index: $end"
             new_length=$((end-start))
             if [ $new_length -ge $MAX ]; then
-              batch=${urls:offset:length}
+              echo "Batch from ${offset} of length ${length}"
+              batch=${urls:$offset:$length}
               echo $batch
               start=$((offset + length))
               length=$((end - start))