Add manual trigger for nightly realase

[matklad]

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.

[Veykril]

We actually have this now I believe?