Integrate gitlab-ci/set-master-branch-manifest-in-jinja2

Commit: seco-ne/yocto/infrastructure/gitlab-ci@d9660a80

build-jobs.jinja2: Specify master branch variable again

If the MASTER_BRANCH_MANIFEST variable is set in the manifest's .gitlab-ci.yml file, it may not find its way to the generated job, if it is not explicitly set during generation. That is done with this patch.