ci: reset prerelease number on release

[anatawa12]

What

Fix https://github.com/misskey-dev/release-manager-actions/issues/13

Why

Additional info (optional)

Checklist

• [ ] Read the contribution guide
• [ ] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [ ] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests

[syuilo]

🙏