Autopkgtest: use /var/tmp/ instead of /tmp/ for debootstrap
/tmp/ is now a tmpfs, and it's mounted nodev, so debootstrap cannot use it:
76s 2024/05/28 08:39:27 Debootstrap | E: Cannot install into target '/tmp/autopkgtest.YDXle7/build.twd/src/.debos-1362844167/root' mounted with noexec or nodev
Switch to /var/tmp/ to fix debci failing since systemd 256~rc3-3
https://ci.debian.net/packages/d/debos/testing/amd64/47065797/
Closes: #1072089