ci: Don't run pipelines for detached heads

By default, merge request pipelines run with detached heads. This is problematic for building the docs using ReadTheDocs because it requires a branch name to map to a version in RTD. But with a detached head, there is no branch.

So let's disable MR pipelines. When pushing changes to a branch that has an MR, the branch pipeline will still run and the result shows up in the MR.

Signed-off-by: Ryan Roberts ryan.roberts@arm.com