multi-arch-builders: force hostname; prune during the night

[dustymabe]

commit 3b8589daaff04eb1c95681641a7602f35f7c7a49
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Tue May 24 09:43:06 2022 -0400

    multi-arch-builders: build and prune at a different time of day.
    
    Rather than 00:00 UTC let's push it further back hopefully when
    most of our team is either still asleep or already asleep. I've
    seen a couple cases recently where the 00:00 UTC runs happen around
    the time the `testing-devel` is running for the day from the
    bump-lockfile bump and it ends up taking out an intermediate container
    and volume used by gangplank.

commit c4d44cfbc77ac96e6cfeeef52de38683751d9cf9
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Tue May 24 09:41:03 2022 -0400

    multi-arch-builders: Force writing /etc/hostname
    
    Force /etc/hostname even if the file already exists. I saw this on
    either the ppc64le or s390x builder where the hostname file had
    already been created.