Keep latest plugin builds available

[davfre]

The build artifacts currently expire after GitHub’s retention period. This keeps the newest successful default-branch packages available in a rolling Latest builds release.

What changes

• After all three packaging jobs succeed on the default branch, download their artifacts and publish stable Linux, Windows, and universal macOS ZIP assets.
• Move the latest-builds tag and update the existing prerelease instead of accumulating releases.
• Keep pull request, feature branch, tag, and manual builds artifact-only.
• Point the README download link at the persistent release page, replacing the Actions and nightly.link alternatives.

Validation

• actionlint .github/workflows/build.yml
• Dry-run of archive discovery, platform naming, and ZIP layout for all three matrix artifacts
• git diff --check

[robbert-vdh]

Thanks for settings this up! I wanted to avoid doing it this way when there was still plenty of activity on the repo's master branch, but with expiring artifacts in GitHub actions this is probably for the best.