gitlab-ci: Fix the pipeline on forks

Our guidelines say that to contribute people should fork repositories,
but at the moment this gives them a broken pipeline.

Tweak things in a way that the main project triggers
tests/apertis-test-cases-web to collect and indirectly publish all the
release branches while forked repositories directly publish their own
branches via Pages for easier testing.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>