gitlab CI: don't run MR pipelines in forks

gitlab CI: don't run MR pipelines in forks

Commit originally by Simon Ser in wayland/wayland-protocols!305.

Currently our CI setup has a downside: for each push on a merge
request, two pipelines are triggered. The first is triggered in
the context of the forked repository, and the second is triggered
in the context of the MR in the parent repository.

Replace the workflow rules with the ones in the official docs [1],
so that a branch pipeline isn't triggered when a MR exists for that
branch.

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

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>