use correct dir for creation of icecream env

use correct dir for creation of icecream env

/var/run might point to the absolute path /run. Thus any commands
referencing $BUILD_ROOT/var/run are actually using the hosts /run
directory, whereas the "cd" tries to change to the nonexisting
/run/icecream directory and fails.

This is a cherry-pick from upstream:
https://github.com/openSUSE/obs-build/commit/21359a6ebcbacdb1754a7c4339a7cca7e496e52f

Change-Id: Ibb87b525f115c5f9435c18db9c72773ca058d8ff
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Julien Isorce <j.isorce@samsung.com>