build: fix Makefile wrt finding node executable

Not all shells set PWD, so use CURDIR instead.

`which node` may return the empty string, so guard against that too. Consider:

if [ -x `which velociraptor` ] && [ -e `which velociprator` ];\
then echo "run"; else echo "keep calm"; fi;

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

build