chore: add e2e test with kcp main builds

[embik]

This is more of an idea than a finished task, so it's up for discussion.

I had the thought that it would be nice if we ran our e2e tests with both the current release tag and the latest main build. This way, we could catch incompatibilities (either upstream or in our code) faster. The job should probably be optional and not block PRs being merged, but the additional feedback of "the operator will work / won't work with the next kcp version as it stands" might be quite useful.