package/util-linux: disable runuser for the host build

package/util-linux: disable runuser for the host build

runuser allows running commands as another user, but needs to run as
root to be able to setuid(). But Buildroot does not require running as
root, and so runuser can't be used.

Incientally, that fixes host build in case unsuitable libs are found on
the system:
    http://lists.busybox.net/pipermail/buildroot/2021-February/304261.html

Reported-by: GA K <guyarkam@gmail.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr:
  - expand the commit log with a more fundamental explanation that
    runuser can't be used anyway
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>