springboot Polish Slack notification when no build scan was published

Hi,

this PR should close #21666 by moving the link to the generated file that is later read when sending the Slack notification.

I have no way of testing this really, so forgive me if things are not working as expected.

Cheers, Christoph

Comment From: wilkinsona

Thanks very much, @dreis2211. We don't really have a good way of testing this either. FWIW, what you've done is what I had in mind. Let's merge this one and see what happens.