Fix make-generate using non-vendor packages (#8008)

Makefile

@@ -41,7 +41,7 @@ endif
 
 LDFLAGS := $(LDFLAGS) -X "main.MakeVersion=$(MAKE_VERSION)" -X "main.Version=$(GITEA_VERSION)" -X "main.Tags=$(TAGS)"
 
-PACKAGES ?= $(filter-out code.gitea.io/gitea/integrations/migration-test,$(filter-out code.gitea.io/gitea/integrations,$(shell $(GO) list ./... | grep -v /vendor/)))
+PACKAGES ?= $(filter-out code.gitea.io/gitea/integrations/migration-test,$(filter-out code.gitea.io/gitea/integrations,$(shell GO111MODULE=on $(GO) list -mod=vendor ./... | grep -v /vendor/)))
 SOURCES ?= $(shell find . -name "*.go" -type f)
 
 TAGS ?=