[CI] Explicitly set artifact expiry time to something sane

Yesterday, ZIVGITLAB ran out of artifact storage space, and we're probably part of the problem. I just talked to the admin, he changed the storage directory so there is some more space, and this isn't of critical urgency anymore. Still, we should configure a shorter expiry time in our jobs (now the instance default is 100 days, was infinite before yesterday). A week or a day would probably be reasonable for our purposes.

This is part of admin#9

Main offenders:

Edited by Dr. Jorrit Fahlke
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