Fixes an issue where the snap workflow failed during the upload artifact step with `Input required and not supplied: path` (see [run 34387324540](https://github.com/canonical/imagecraft/actions/runs/34387324540/job/102587117930)). The `canonical/craft-actions/snapcraft/pack` action exposes artifact outputs using plural naming (`snaps`), whereas the previous action used singular `snap`. This caused `${{ steps.build-imagecraft.outputs.snap }}` to evaluate to an empty string, causing `actions/upload-artifact` and `canonical/action-publish` to receive empty paths. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/imagecraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing.