Add manual trigger for nightly realase #9283
Description
At the moment, a nightly release can be started by pushing to the trigger-nightly branch. The reason for this is historical -- this functionality had been implemented before github actions added "trigger workflow manually" feature. We probably should switch to the manual triggers, so that a new release can be conveniently kicked directly from a PR. We need to verify that a manual trigger would still allow to trigger on arbitrary commit though.