backend: Explicitly name "backend" as the default make goal

setup-env.mk is included before the "backend" goal is defined and
because we were not explicitly naming it as the default goal, the
1st goal in setup-env.mk became the implicit default goal.

backend/Makefile

@@ -4,6 +4,8 @@ COMMIT ?= $(shell git rev-parse --short=7 HEAD)
 ARO_HCP_BASE_IMAGE ?= ${ARO_HCP_IMAGE_ACR}.azurecr.io
 ARO_HCP_BACKEND_IMAGE ?= $(ARO_HCP_BASE_IMAGE)/arohcpbackend:$(COMMIT)
 
+.DEFAULT_GOAL := backend
+
 backend:
 	go build -o aro-hcp-backend .
 .PHONY: backend