kselftest: Fix NULL INSTALL_PATH for TARGETS runlist

As per commit 131b30c94fbc ("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: 131b30c94fbc ("kselftest: exclude failed TARGETS from runlist")
Signed-off-by: Prabhakar Kushwaha <[e-mail]>
Reviewed-by: Cristian Marussi <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 	@# included in the generated runlist.
 	for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
-		[ ! -d $$INSTALL_PATH/$$TARGET ] && echo "Skipping non-existent dir: $$TARGET" && continue; \
+		[ ! -d $(INSTALL_PATH)/$$TARGET ] && echo "Skipping non-existent dir: $$TARGET" && continue; \
 		echo "[ -w /dev/kmsg ] && echo \"kselftest: Running tests in $$TARGET\" >> /dev/kmsg" >> $(ALL_SCRIPT); \
 		echo "cd $$TARGET" >> $(ALL_SCRIPT); \
 		echo -n "run_many" >> $(ALL_SCRIPT); \