.gitlab-ci.yml: define top-level workflow

It seems that defining rules with the "extends" mechanism is not working
very well. We may have better luck by using the workflow definition [1],
which incorporates our rules and automatically applies to all the jobs
in the pipeline.

[1] https://docs.gitlab.com/ee/ci/yaml/#workflow



Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>