ci: Avoid duplicate pipelines for MRs on meta layers

We already have this in yocto-ktn-build.yml and want to apply the same to meta layer pipelines.

Do not run branch pipelines when there is an open MR for this branch. This prevents duplicate piplines (MR & push) for those cases. See:

https://docs.gitlab.com/ee/ci/yaml/workflow.html#switch-between-branch-pipelines-and-merge-request-pipelines