ci: only release preview on manual dispatch from main or v2

[egil]

Pull request description

PR meta checklist

• Pull request is targeted at main branch for code
  or targeted at stable branch for documentation that is live on bunit.dev.
• Pull request is linked to all related issues, if any.
• I have read the CONTRIBUTING.md document.

Code PR specific checklist

• My code follows the code style of this project and AspNetCore coding guidelines.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the appropriate sub section in the CHANGELOG.md.
• I have added, updated or removed tests to according to my changes.
  • All tests passed.