execute: Switch back to chroot()

execute: Switch back to chroot()

pivot_root() seems to be very complicated to use and will require us to
have the container run on a different file system. That is however not
possible when Pakfire is running as an un-privileged user.

Since pivot_root() does not seem to offer any advantages over chroot(),
we switch back to chroot() which is easier to use.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>