So, answering myself.
On Xcode 14.2.0, perhaps somewhat earlier, pkgbuild has an option –min-os-version which does the job.
Otherwise, my theory is that pkgbuild looks into the bundled code including dependencies and uses the lowest link level available in all of them, which in my case is 12. And there is just no way around this without the option available in 14.2.0.