"make push-multiarch" matched both push-multiarch and push-%.  This
seems to be none-deterministic and in at least one
repo (external-provisioner), make picked the wildcard rule which then
failed because there is no "multiarch" command.

This ambiguity gets resolved by instantiating the wildcard rules only
for existing commands. The advantage also is that "make
push-no-such-command" will fail with an obvious "No rule to make
target 'push-no-such-command'" instead of attempting to build the
command.