Are not published workshop papers considered in similarity check for an extended journal paper? I am going to submit my journal paper in 2 weeks. Besides, there is a workshop whose deadline is in a couple of days. I am going to submit a brief version of my potential journal paper (12 pages) in this workshop (4 pages).

In the workshop website it's mentioned that "Please note the papers will not be published in a proceedings but will be made available via this website."

Now I'm concerned about one thing. As the workshop papers will be made available via this website, I assume they will be considered in the similarity check for my extended journal paper submission.

Will not that cause my journal paper to be considered as plagiarism (or self-plagiarism)?
Just water it down for the conference.  Basically do a version of your slides.  Also all they (likely) need now for the meeting is your abstract.  Send that in and figure out paper later.