Makefile: Fix microk8s image target
Makefile: Fix microk8s image target

Recent commits broke the microk8s build target by renaming the default
image. Fix it up by referring to the new common variable names used to
construct docker image names.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tklauser/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium