Integrate gitlab-ci/fix-build-merge-request-job and 1 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@7f68f307

Yocto manifest build: re-enable build:merge_request job

This job was accidentally disabled in 2e6c1b4, because it was included
in the generated child pipeline. This pipeline is not created on the
master, though, so the condition for the job was never met.

Move the job to the parent pipeline again, so that it gets executed on
the master like before.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@970c7fd7

Re-add yamllint stage to ci test manifest pipeline

Was accidentally removed in previous commit.