build: Fetch KUBECTL digests on-demand

Unfortunately, the current renovate-config does not support the sourcing of digests for
kubectl. To bypass this limitation while keeping the auto bumps working, the digest will
be sourced on demand based on the dl.k8s.io endpoint.

hack/make/deps.mk

@@ -2,12 +2,9 @@
 HELM_VERSION := v3.13.3-rancher1
 
 KUBECTL_VERSION := v1.26.9
-# curl -L "https://dl.k8s.io/release/$KUBECTL_VERSION/bin/linux/arm64/kubectl.sha256"
-KUBECTL_SUM_arm64 := f945c63220b393ddf8df67d87e67ff74b7f56219a670dee38bc597a078588e90
-# curl -L "https://dl.k8s.io/release/$KUBECTL_VERSION/bin/linux/amd64/kubectl.sha256"
-KUBECTL_SUM_amd64 := 98ea4a13895e54ba24f57e0d369ff6be0d3906895305d5390197069b1da12ae2
-# curl -L "https://dl.k8s.io/release/$KUBECTL_VERSION/bin/linux/s390x/kubectl.sha256"
-KUBECTL_SUM_s390x := 6c3f1cac8d70286eb4a661d783558101b9e4891e7997f744183506466a03625f
+KUBECTL_SUM_arm64 ?= $(shell curl -L "https://dl.k8s.io/release/v$(KUBECTL_VERSION)/bin/linux/arm64/kubectl.sha256")
+KUBECTL_SUM_amd64 ?= $(shell curl -L "https://dl.k8s.io/release/v$(KUBECTL_VERSION)/bin/linux/amd64/kubectl.sha256")
+KUBECTL_SUM_s390x ?= $(shell curl -L "https://dl.k8s.io/release/v$(KUBECTL_VERSION)/bin/linux/s390x/kubectl.sha256")
 
 # renovate: datasource=github-release-attachments depName=kubernetes-sigs/kustomize
 KUSTOMIZE_VERSION := v5.3.0