WIP: PoC Demo: mvn multi-module no-compilation SAST stage

What

PoC of a no-compile SAST execution leveraging existing libs as `build artifacts.

This demo shows the usage of the new MAVEN_REPO_PATH ENV flag to specify a local maven repository, as passed to the mvn package command.

See PoC issue https://gitlab.com/gitlab-org/gitlab-ee/issues/8619 and parent issue https://gitlab.com/gitlab-org/gitlab-ee/issues/8212