ci: use detached CI pipelines

See [1], required to allow contributors to trigger CI pipelines
for MRs. Example failure can be found at [2].

[1]: freedesktop/freedesktop#540
[2]: https://gitlab.freedesktop.org/i509VCB/wayland-protocols/-/jobs/40117393



Signed-off-by: Simon Ser <contact@emersion.fr>