feature: add s390x builds

[skuethe]

I would like to add s390x builds to the in-place automation, so that images can be used on zLinux.
The code changes should be minimal (see #1078 as a reference).

I will go ahead and create a PR for this.

Just FYI:
Your CONTRIBUTING.md says that each PR needs an corresponding issue, but your issue template for feature requests is redirecting to the discussion board.

[skuethe]

PR: #1097