Makefile: Ensure GO111MODULE=on is set

Makefile

@@ -2,6 +2,8 @@
 PKG  := $(shell go list .)
 PKGS := $(shell go list ./...)
 
+GO111MODULE=on
+
 .PHONY: bootstrap
 bootstrap: testdata # set up the project for development
 
@@ -11,20 +13,20 @@ lint: # lints the package for common code smells
 		out=`gofmt -s -d $$f`; \
 		test -z "$$out" || (echo $$out && exit 1); \
 	done
-	GO111MODULE=on go run golang.org/x/lint/golint -set_exit_status $(PKGS)
+	go run golang.org/x/lint/golint -set_exit_status $(PKGS)
 	go vet -all -shadow -shadowstrict $(PKGS)
 
 .PHONY: quick
 quick: testdata # runs all tests without the race detector or coverage
-	GO111MODULE=on go test $(PKGS)
+	go test $(PKGS)
 
 .PHONY: tests
 tests: testdata # runs all tests against the package with race detection and coverage percentage
-	GO111MODULE=on go test -race -cover $(PKGS)
+	go test -race -cover $(PKGS)
 
 .PHONY: cover
 cover: testdata # runs all tests against the package, generating a coverage report and opening it in the browser
-	GO111MODULE=on go test -race -covermode=atomic -coverprofile=cover.out $(PKGS) || true
+	go test -race -covermode=atomic -coverprofile=cover.out $(PKGS) || true
 	go tool cover -html cover.out -o cover.html
 	open cover.html
 
@@ -78,13 +80,13 @@ testdata-go: bin/protoc-gen-go bin/protoc-gen-debug # generate go-specific testd
 		testdata-outputs
 
 bin/protoc-gen-go: # creates the protoc-gen-go plugin using the vendored version
-	GO111MODULE=on go build -o $@ github.com/golang/protobuf/protoc-gen-go
+	go build -o $@ github.com/golang/protobuf/protoc-gen-go
 
 bin/protoc-gen-example: # creates the demo protoc plugin for demonstrating uses of PG*
-	GO111MODULE=on go build -o $@ ./testdata/protoc-gen-example
+	go build -o $@ ./testdata/protoc-gen-example
 
 bin/protoc-gen-debug: # creates the protoc-gen-debug protoc plugin for output ProtoGeneratorRequest messages
-	GO111MODULE=on go build -o $@ ./protoc-gen-debug
+	go build -o $@ ./protoc-gen-debug
 
 .PHONY: clean
 clean: