.gitlab-ci.yml: do runtime tests only on explicit trigger

When the runtime tests were first introduced, they still ran pretty
quickly. Nowadays, however, there are a lot of runtime tests, and some
of them take a really long time. So running them on every push is
really too much.

Just like we do for the defconfigs, run them on explicit trigger only.

The explicit trigger is now done every week, but it can be increased
to e.g. twice or three times per week.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>