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.

assigned to @jonas.hoeppner

merged

Integration merge request: https://git.seco.com/seco-ne/yocto/infrastructure/ci-test/minimal-foo/-/merge_requests/168

Integration merge request: https://git.seco.com/seco-ne/yocto/infrastructure/ci-test/minimal-bar/-/merge_requests/172

Integration merge request: seco-ne/3rd-party/kuk/uboot-imx-kuk!46 (merged)

mentioned in merge request seco-ne/3rd-party/kuk/uboot-imx-kuk!46 (merged)

Integration merge request: seco-ne/kernel/linux-guf!109 (merged)

mentioned in merge request seco-ne/kernel/linux-guf!109 (merged)

Integration merge request: https://git.seco.com/seco-ne/yocto/infrastructure/ci-test/minimal-srcrev/-/merge_requests/31

Integration merge request: seco-ne/kernel/linux-imx-kuk!89 (merged)

mentioned in merge request seco-ne/kernel/linux-imx-kuk!89 (merged)

Integration merge request: seco-ne/kernel/modules/egalaxi2c!36 (merged)

Integration merge request: https://git.seco.com/seco-ne/yocto/infrastructure/ci-test/minimal-manifest/-/merge_requests/390

mentioned in merge request seco-ne/kernel/modules/egalaxi2c!36 (merged)

Integration merge request: seco-ne/kernel/modules/gfplatdetect!32 (merged)

mentioned in merge request seco-ne/kernel/modules/gfplatdetect!32 (merged)