[FIX] Add npm, yarn executable

Makefile

@@ -15,16 +15,11 @@ build:
 install.dep:
 ifeq ($(shell command -v yarn),)
 	@echo "Installing yarn..."
-
-	$(eval npm  := $(shell command -v npm))
-	$(npm) install -g yarn
+	npm install -g yarn
 endif
 
-	$(eval yarn := $(shell command -v yarn))
-
 install: install.dep
-	$(yarn)										\
-		--cwd $(BASEDIR)
+	yarn --cwd $(BASEDIR)
 
 test:
 	NODE_ENV=test								\