Makefile: Use target-specific vars for microk8s
Makefile: Use target-specific vars for microk8s

The microk8s target previously relied on an env var declared at the
top-level of the Makefile, which will conflict with an upcoming commit.
Move it into a target-specific variable. This also means we can remove
the redundant tagging operation.

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

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium