Admittedly this is a bit of necromancy as this question is over a year old but...
I don't think you can do this in the multibranch pipeline. The description of the script path reads:
Relative location within the checkout of your Pipeline script. Note
that it will always be run inside a Groovy sandbox. Default is
Jenkinsfile if left empty. (just use checkout scm to retrieve sources
from the same location as is configured here)
Since this question was posted some changes have been made to jenkins so that it only clones the repo if you are a person who is allowed to make changes to the Jenkins script (you can configure your jenkins as to how it determines this). Which is great for people who aren't allowed to make changes to the jenkins file as they skip the initial clone and checkout. Of course if you are allowed to make changes, this doesn't help much. And there is a potential issue in that someone can make a change to the Jenkins script but the changed script wont get run until it is merged into the master branch, and if there are any errors, it's too late...