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

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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.