Fix empty GOPATH behaviour in Makefile (#8724)

Makefile

@@ -43,7 +43,7 @@ else
 endif
 
 # Golang Flags
-GOPATH ?= $(GOPATH:):./vendor
+GOPATH ?= $(shell go env GOPATH)
 GOFLAGS ?= $(GOFLAGS:)
 GO=go
 GO_LINKER_FLAGS ?= -ldflags \
@@ -273,7 +273,7 @@ gofmt: ## Runs gofmt against all packages.
 
 store-mocks: ## Creates mock files.
 	go get github.com/vektra/mockery/...
-	GOPATH=$(shell go env GOPATH) $(shell go env GOPATH)/bin/mockery -dir store -all -output store/storetest/mocks -note 'Regenerate this file using `make store-mocks`.'
+	$(GOPATH)/bin/mockery -dir store -all -output store/storetest/mocks -note 'Regenerate this file using `make store-mocks`.'
 
 update-jira-plugin: ## Updates Jira plugin.
 	go get github.com/mattermost/go-bindata/...