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>