More Makefile enhancements (#569)

Allows us to make use of of make's target deps instead of re-execing
make in our build target just for custom, one-shot environment changes.

Keeps e2e bin in bin/e2e/virtual-kubelet.

Makefile.e2e

@@ -20,6 +20,12 @@ skaffold/%: skaffold.validate
 		-f $(PWD)/hack/skaffold/virtual-kubelet/skaffold.yml \
 		-p $(PROFILE)
 
+bin/e2e:
+	@mkdir -p bin/e2e
+
+bin/e2e/virtual-kubelet: bin/e2e
+	GOOS=linux GOARCH=amd64 $(MAKE) OUTPUT_DIR=$(@D) build
+
 # e2e runs the end-to-end test suite against the Kubernetes cluster targeted by the current kubeconfig.
 # It automatically deploys the virtual-kubelet with the mock provider by running "make skaffold MODE=run".
 # It is the caller's responsibility to cleanup the deployment after running this target (e.g. by running "make skaffold MODE=delete").
@@ -30,10 +36,9 @@ e2e: NODE_NAME := vkubelet-mock-0
 e2e: TAINT_KEY := virtual-kubelet.io/provider
 e2e: TAINT_VALUE := mock
 e2e: TAINT_EFFECT := NoSchedule
-e2e: tags_with_mock := $(VK_BUILD_TAGS) mock
-e2e: e2e.clean
-	GOOS=linux GOARCH=amd64 $(MAKE) VK_BUILD_TAGS="$(tags_with_mock)" build; \
-	$(MAKE) skaffold/run; \
+e2e: export VK_BUILD_TAGS += mock_provider
+e2e: e2e.clean bin/e2e/virtual-kubelet skaffold/run
+	@echo Running tests...
 	cd $(PWD)/test/e2e && go test -v -tags e2e ./... \
 		-kubeconfig=$(KUBECONFIG) \
 		-namespace=$(NAMESPACE) \
@@ -43,5 +48,7 @@ e2e: e2e.clean
 		-taint-effect=$(TAINT_EFFECT)
 
 .PHONY: e2e.clean
+e2e.clean: NODE_NAME ?= vkubelet-mock-0
 e2e.clean: skaffold/delete
-	kubectl delete --ignore-not-found node $(NODE_NAME)
+	kubectl delete --ignore-not-found node $(NODE_NAME); \
+	if [ -f bin/e2e/virtual-kubelet ]; then  rm bin/e2e/virtual-kubelet; fi