kselftest: Fix NULL INSTALL_PATH for TARGETS runlist

As per commit 131b30c9 ("kselftest: exclude failed TARGETS from
runlist") failed targets were excluded from the runlist. But value
$$INSTALL_PATH is always NULL. It should be $INSTALL_PATH instead
$$INSTALL_PATH.

So, fix Makefile to use $INSTALL_PATH.

Fixes: 131b30c9 ("kselftest: exclude failed TARGETS from runlist")
Signed-off-by: Prabhakar Kushwaha <pkushwaha@marvell.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>