simplify construction of "kustomizeImages"

[krancour]

Requires change similar to akuity/kargo#1481

Here in Kargo Render, the kustomize binary isn't invoked directly, but the same complication removed by akuity/kargo#1481 can be removed in Kargo Render's use of the Argo CD repo server.

func Render(
	ctx context.Context,
	path string,
	images []string,
) ([]byte, error) {
	kustomizeImages := make(argoappv1.KustomizeImages, len(images))
	for i, image := range images {
		addr, _, _ := strings.SplitLast(image, ":")
		kustomizeImages[i] =
			argoappv1.KustomizeImage(fmt.Sprintf("%s=%s", addr, image))
	}
...