Adds additional tag without -ckN suffix

Currently, we publish manifest lists for each image and version with the
-ckN suffix, where N is the number of versions we have published for
that rock.

This commit also publishes the same manifest lists without the -ckN
suffix. This will help in 2 regards:

- We typically want to deploy the latest version of a published rock.
  When it comes to user experience, it's a lot easier to just use the
  ghcr.io/canonical/foo:1.0.0 rock, rather than ghcr.io/canonical/foo:1.0.0-ckN,
  where a user would have too manually check what is the latest N
  revision of that rock. This issue is even more egregious in cases in
  which we'd need multiple rocks for the same application deployment
  (e.g.: Longhorn).
- The documentation where these rocks are referenced with the -ckN
  suffix will get outdated over time, referencing an out-of-date revision.
  Instead, the manifest list without the -ckN suffix will always point
  towards the latest revision.

The -ckN manifest lists are still published, so a user may still be able
to pick a previous revision if needed.

.github/workflows/assemble_multiarch_image.yaml

@@ -83,4 +83,8 @@ jobs:
           const dryRun = ${{ inputs.dry-run }}
           const rockMetas = JSON.parse(`${{ steps.assemble-image-tags.outputs.rock-metas }}`)
           ${{ steps.create-and-push-manifest-js.outputs.content }}
-          await main(rockMetas, registry, dryRun)
+          await main(rockMetas, registry, dryRun)
+
+          // Create and push manifests for versions without the -ckN suffix.
+          const metas = JSON.parse(`${{ inputs.rock-metas }}`)
+          await main(metas, registry, dryRun)