CI: Optimize pipeline for merge train setup

Doing the ohos build test with oniro-openharmony-toolchain and oniro-openharmony-bundle images is rather expensive, so we will not run them automatically until the final merge train pipeline.

This way we can still ensure that we don't merge anything that breaks this, while allowing faster pipeline execution during branch and merge request iterations.

Related to eclipse/oniro-core/meta-openharmony#62