This showed up with dnf in containers when TMPDIR was set; dnf started
trying to write to this directory while in the chroot.
We already do stripping like this in run_in_target -- but this is a
bit of a unique place because it's actually setting up the initial
chroot so the target doesn't actually exist yet; so we just hard-code
it in place here.
Change-Id: If7310cb820846da903bf60daa4486c8bf7cb0136
df3ad26f58