lmbench: fix the CONFIG path in lmbench-run

The 'config' script which gets run returns '../scripts/CONFIG.<machine>', so
the current value was '${datadir}/lmbench/../scripts/CONFIG.<machine>', which
is obviously not ideal. Remove the $SHAREDIR prefix, as the relative path will
work from the scripts dir where lmbench-run runs. With this, we can install
a custom CONFIG.qemux86 into ${datadir}/lmbench/scripts/, for example.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>