rootfs-postcommands.bbclass: move host-user-contaminated.txt to ${S}

This is to ensure host-user-contaminated.txt would be removed before
do_rootfs runs, since ${S} is in cleandirs of do_rootfs, otherwise, a
host-user-contaminated.txt file that generated from previous builds
could be used which is wrong.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>