package/nfs-utils: fix nfs-utils_env.sh path

Per upstream commit bf12e1fa9f48e8ee91a2869e69484c70595b5160
https://cdn.kernel.org/pub/linux/utils/nfs-utils/1.3.4/1.3.4-Changelog

Currently nfs-utils_env.sh is installed in a
directory that is own by systemd. They requested
via https://bugzilla.redhat.com/show_bug.cgi?id=1303113


to move the script into /usr/libexec/nfs-utils

As such we must update the installation path for nfs-utils_env.sh.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Tested-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>