gitlab CI: increase the artifacts expiry to 6h

20 min for the "this docker image is ok" marker should be enough but not when
we're hit with random stuck containers in the next stage. By the time those
time out the artefacts have been removed and we now get a dependency error,
forcing us to re-run the whole pipeline.

Since the marker is only a few bytes, we can keep this for a bit longer
without risking running out of space.

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