Improve root project version in nexus-publish-e2e-multi-project

[szpak]

The root project version in nexus-publish-e2e-multi-project is reported:

:nexus-publish-e2e-multi-project:unspecified

This breaks e2e testing for findStagingRepository (#145) in that project (disabled until it is fixed). It's the multi-module Kotlin project with the version logic set in buildSrc/library-publishing-conventions.gradle.kts, applied just on the submodules. The version logic could be extracted and applied also on the root project. Or any other smart approach could be used.