Makefile: Fix gopath in command (#21)

Fixes build failures in some environments. 
diff --git a/Makefile b/Makefile
index d32b6be..235ec03 100644
--- a/Makefile
+++ b/Makefile
@@ -36,7 +36,7 @@
 
 .PHONY: all
 all: fmt vendor | $(BASE) ; $(info $(M) Building executable…) @ ## Build program binary
-	$Q cd $(BASE) && $(GO) build \
+	$Q cd $(BASE) && GOPATH=$(GOPATH) $(GO) build \
 		-tags release \
 		-ldflags '-s -w -X $(PACKAGE)/cmd.Version=$(VERSION) -X $(PACKAGE)/cmd.BuildDate=$(DATE)' \
 		-o bin/$(PACKAGE) cmk.go