verify sha256sum of kubectl

This patch extends the installation of the kubectl binary to validate
the checksum of the downloaded file, similar to what is already done for
kind. This approach is also described as an optional step on https://kubernetes.io/docs/tasks/tools/install-kubectl-linux/#install-kubectl-binary-with-curl-on-linux.

Author: felix-kaestner

kind.sh

@@ -227,8 +227,12 @@ install_kubectl() {
 
     mkdir -p "${kubectl_dir}"
 
-    curl -sSLo "${kubectl_dir}/kubectl" "https://dl.k8s.io/release/${kubectl_version}/bin/linux/${arch}/kubectl"
-    chmod +x "${kubectl_dir}/kubectl"
+    pushd "${kubectl_dir}"
+    curl -sSLo kubectl "https://dl.k8s.io/release/${kubectl_version}/bin/linux/${arch}/kubectl"
+    curl -sSLo kubectl.sha256 "https://dl.k8s.io/release/${kubectl_version}/bin/linux/${arch}/kubectl.sha256"
+    echo "$(cat kubectl.sha256) kubectl" | sha256sum -c
+    chmod +x kubectl
+    popd
 }
 
 create_config_with_registry() {