build: cache-args still documents a per-call cache tag that nothing passes #4504

The optional second argument of cache-args in hack/common-envs.mk has no callers left. Its comment says to pass an explicit cache tag "for images that build a distinct artifact per loop iteration (e.g. ubuntu-container-disk per k8s ver)", but no package Makefile builds ubuntu-container-disk any more, and no Makefile calls cache-args with a second argument.

It matters once the arm64 CI leg lands, because that leg separates its cache by the default tag (buildcache vs buildcache-arm64). A per-call tag passed as the second argument would bypass that, so amd64 and arm64 builds of such an image would write the same cache ref.

Either drop the second argument and its comment, or make an explicit tag carry the arch suffix too. Low severity, nothing uses it today.