Does your build otherwise not resolve remote dependencies, not even from a local Nexus or Artifactory? If you use a local artifact repository you can place the build scan plugin there if you want and configure your builds to fetch plugins from that repository.
https://docs.gradle.org/current/userguide/plugins.html#sec:custom_plugin_repositories
I placed the jar in a local directory as for other thirdyparty dependencies
If I’m interpreting this right you are saying all your dependencies are checked-in as JARs in VCS. If so then yes, putting the build-scan plugin in there as well will be fine.