fix: nightly dispatch creates today's tag when retrying a failed nightly

This PR fixes the `workflow_dispatch` path for nightly releases. Previously,
when a scheduled nightly failed (so no tag was created) and someone manually
re-triggered the workflow, it would find the most recent existing nightly tag
(from a previous day) and create a `-revK` revision of that old tag. Now it
checks if today's nightly tag exists: if not, it creates it directly; if it
already exists, it creates a `-revK` revision as before.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>