The build-iso workflow used softprops/action-gh-release@v2 unconditionally,
which only speaks the GitHub Releases REST API. When the workflow runs on
the Forgejo runner registered on nullstone, those steps would fail.
Add a server_url check so the GH-only path runs only on github.com, and
mirror it with a curl-based step that hits the Forgejo /api/v1/releases
endpoints. Behaviour:
- github.com: identical to before (action-gh-release@v2).
- git.s8n.ru: drop+recreate ci-latest release, upload chunked assets
via the Forgejo attachments API.
Tag-driven "Attach to release" path mirrored the same way.
Refs: A1 build-eng task — Forgejo runner adaptation.
