/kind bug

What steps did you take and what happened:
Quote from #146 (comment):

The only thing what is a little annoying is that the command doesn't detect if there's already the same release in the registry and uploads them every time. Usually I run the csctl create many times to get the generated hash out of it to create the ClusterStack yamls automatically. With csctl publish it always pushes it again with a new digest. Would it make sense to check on the locally generated hash and if it's already there it shouldn't be pushed? At least when using hash mode that should be unique.

What did you expect to happen:
Don't push assets when the local generated hash string is identical.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• csctl version: (use csctl version)
• OS (e.g. from /etc/os-release):