Mock: refine go generate command

mocks.go

@@ -1,6 +1,6 @@
 package core
 
-//go:generate go get -u github.com/golang/mock/gomock
+//go:generate go install github.com/golang/mock/gomock
 //go:generate go install github.com/golang/mock/mockgen
 
 //go:generate mockgen -package mocks -destination testing/mocks/io.go -mock_names Reader=Reader,Writer=Writer io Reader,Writer