The `artifact-url` ouput [returned by the upload-artifact@v4 action](https://code.forgejo.org/actions/upload-artifact#outputs) can be used to download the artifact. It was previously 404. To implement this compatibility fix, the web UI URL to download artifacts (i.e. `/{owner}/{repo}/actions/runs/{run_id}/artifacts/{artifact_name}`) now relies on an identifier that is unique accross the instance. URLs to download artifacts that were bookmarked or copied prior to this change use an id relative to the repository and will no longer work. It previously was `/{owner}/{repo}/actions/runs/{run_index}/artifacts/{artifact_name}`, note the difference between `{run_id}` and `{run_index}`. The new URL can be obtained again by visiting the parent page, which still uses the relative id (`/{owner}/{repo}/actions/runs/{run_index}`).
