Exporting multiple image types

[supakeen]

osbuild does caching quite well, especially when exporting multiple pipelines from a single manifest.

We should take advantage of this and allow selecting multiple image types in image-builder build. That can either be done positionally or together with a change like #31.

#43 is probably also relevant here.

[github-actions]

This issue is stale because it had no activity for the past 365 days. Remove the "Stale" label or add a comment, otherwise this issue will be closed in 30 days.

[supakeen]

Not stale.