fix: k8s_setup.sh should use newer kubectl release URL

[polarathene]

Details from troubleshooting a CI failure: coredns/coredns#7659 (comment)

Changes:

• Use the newer release URL for kubectl.
• Quote wrap variables with user supplied values.

---

Reminder, there's a Nov 2024 PR which might have been helpful in troubleshooting the cause more clearly: #161 (UPDATE: Prefer improvement from #168)

[polarathene]

I'm not sure how my PR is causing the failures raised below 🤷‍♂️

EDIT: I see recent commits merged to master have CI failures, so they're just being ignored and this PR is not to blame.

Reference - Failure logs

external-plugin-tests

$ cd ~/go/src/${CIRCLE_PROJECT_USERNAME}/ci/test/kubernetai
$ go mod tidy
$ GO111MODULE=on go test -v ./...

# ...

# [github.com/coredns/ci/test/kubernetai]
./autopath_test.go:129:12: non-constant format string in call to (*testing.common).Fatalf
FAIL    github.com/coredns/ci/test/kubernetai [build failed]
FAIL

ci/test/kubernetai/autopath_test.go

Lines 127 to 130 in c05e817

	err = kubernetes.WaitForClientPodRecord(namespace)
	if err != nil {
	t.Fatalf(err.Error())
	}

---

k8s-deployment-tests

$ mkdir -p ~/go/src/${CIRCLE_PROJECT_USERNAME}/deployment
$ git clone https://github.com/${CIRCLE_PROJECT_USERNAME}/deployment ~/go/src/${CIRCLE_PROJECT_USERNAME}/deployment
$ mkdir -p ~/go/src/${CIRCLE_PROJECT_USERNAME}/coredns
$ git clone https://github.com/${CIRCLE_PROJECT_USERNAME}/coredns ~/go/src/${CIRCLE_PROJECT_USERNAME}/coredns
$ cd ~/go/src/${CIRCLE_PROJECT_USERNAME}/ci/test/k8sdeployment
$ go mod tidy
$ GO111MODULE=on go test -v ./...

# ...

# [github.com/coredns/ci/test/k8sdeployment]
./configmap_test.go:76:12: non-constant format string in call to (*testing.common).Fatalf
FAIL    github.com/coredns/ci/test/k8sdeployment [build failed]
FAIL

ci/test/k8sdeployment/configmap_test.go

Lines 75 to 77 in c05e817

	if strings.Compare(corefileTranslated, corefileExpected) != 0 {
	t.Fatalf("failed test: Translation does not match.\nGOT:\n" + corefileTranslated + "\n\nEXPECTED:\n" + corefileExpected)
	}