Use modules from vendor dir on build

The build fails when running in a secured environment, without access to the outside world,
as it tries to reach sources such as github.com, k8s.io, etc.

Dockerfile

@@ -1,4 +1,4 @@
-FROM registry.svc.ci.openshift.org/openshift/release:golang-1.10 AS builder
+FROM registry.svc.ci.openshift.org/openshift/release:golang-1.11 AS builder
 WORKDIR /go/src/github.com/openshift/openshift-state-metrics
 COPY . .
 RUN make build

Makefile

@@ -40,7 +40,7 @@ doccheck:
 gofmtcheck:
 	@go fmt $(PKGS) | grep ".*\.go"; if [ "$$?" = "0" ]; then exit 1; fi
 build: clean
-	GO111MODULE=on GOOS=$(shell uname -s | tr A-Z a-z) GOARCH=$(ARCH) CGO_ENABLED=0 go build -ldflags "-s -w -X ${PKG}/version.Release=${TAG} -X ${PKG}/version.Commit=${Commit} -X ${PKG}/version.BuildDate=${BuildDate}" -o openshift-state-metrics
+	GO111MODULE=on GOOS=$(shell uname -s | tr A-Z a-z) GOARCH=$(ARCH) CGO_ENABLED=0 go build -mod=vendor -ldflags "-s -w -X ${PKG}/version.Release=${TAG} -X ${PKG}/version.Commit=${Commit} -X ${PKG}/version.BuildDate=${BuildDate}" -o openshift-state-metrics
 test-unit: clean build
 	GO111MODULE=on GOOS=$(shell uname -s | tr A-Z a-z) GOARCH=$(ARCH) $(TESTENVVAR) go test --race $(FLAGS) $(PKGS)