Integrate gitlab-ci/switch-to-mr-pipelines and 3 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@d67898a2

Manifest: skip build if "skip build" label is set

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@bffc6097

Manifest: switch from branch pipelines to merge request pipelines

This will make the manifest project support the "skip build" label.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@5ba1ac68

Revert "Skip build if "skip build" label is set"

This reverts commit 4db257d36052c74eabed7ef43a05d98d3998ffec.

It does not work on the manifest, because we do not run merge request
pipelines there, so CI_MERGE_REQUEST_LABELS is not set at all.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@4db257d3

Skip build if "skip build" label is set