Add command line option to setup image only

[LinqLover]

We (@fniephaus and me) identified the need for such a feature in hpi-swa/setup-smalltalkCI#9:

I could speed up my Actions workflow if this critical section could be reduced by breaking the smalltalk ci job down into separate jobs for setup + testing.

This sounds like a feature request for smalltalkCI, probably something like a --setup-only cmd line option.

Originally posted by @fniephaus in https://github.com/hpi-swa/setup-smalltalkCI/issue_comments/706032958

[LinqLover]

This would also be helpful for local testing: If you could reuse an already prepared image with your project loaded, it would be easier to inspect it again, without starting the entire process again from the beginning.