CI: optimize packaging tests

Close #541 (closed).

client/.packaging/test.sh

 #!/bin/sh
 # SPDX-License-Identifier: GPL-3.0-or-later
-test -e /usr/sbin/kresc
-/usr/sbin/kresc  # command will fail because of invalid parameters
+test -e sbin/kresc
+sbin/kresc  # command will fail because of invalid parameters
 test "$?" -eq 1  # linker error would have different exit code