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