Group Vulnerability Report CSV Export finalisation duplication

Finalisation currently repeats for every export segment. If the report contains 200 000 vulnerabilities, this will result in 4 export segments and enqueue 4 finalisation jobs. All 4 will execute, meaning we join the export segments 4 times and save the result. The result is idempotent, so this doesn't affect the output of the feature, however because this operation is somewhat expensive (sometimes a minute to do), doing it 4 times can consume a lot of our async execution resource.