Makefile: Add NO_VALGRIND to pytest environment

Previously, by uncommenting NO_VALGRIND in Makefile, the variable
was not propagated and the pytest children processes would still
try to use Valgrind.

Makefile

@@ -215,7 +215,7 @@ ifndef PYTEST
 	@echo "py.test is required to run the integration tests, please install using 'pip3 install -r tests/requirements.txt'"
 	exit 1
 else
-	PYTHONPATH=contrib/pylightning:$$PYTHONPATH TEST_DEBUG=1 DEVELOPER=$(DEVELOPER) $(PYTEST) tests/ $(PYTEST_OPTS)
+	PYTHONPATH=contrib/pylightning:$$PYTHONPATH TEST_DEBUG=1 DEVELOPER=$(DEVELOPER) NO_VALGRIND=$(NO_VALGRIND) $(PYTEST) tests/ $(PYTEST_OPTS)
 endif
 
 # Keep includes in alpha order.