Product image selection: only override version for PR builds

[nightkr]

Description

Originally introduced in #629.

Also adds logging because this was pretty confusing to me.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

[nightkr]

I would have never done that, but I guess there could be reasons for that ;)

I wanted to run the whole test suite against my custom build. Given the choice between changing each individual test to refer to it or changing the default, guess which one is less annoying.. :P

[sbernauer]

That's clever! ;)