Refactor deploy-to-preview job to set explicit kubectx

[meysholdt]

Right now, we set the kubectx config for k3s in VMs to be the default:

gitpod/.werft/jobs/build/deploy-to-preview-environment.ts

Lines 122 to 125 in 3d4e0e4

	VM.copyk3sKubeconfig({ name: destname, path: 'k3s.yml', timeoutMS: 1000 * 60 * 3, slice: vmSlices.KUBECONFIG })
	// NOTE: This was a quick have to override the existing kubeconfig so all future kubectl commands use the k3s cluster.
	// We might want to keep both kubeconfigs around and be explicit about which one we're using.s
	exec(`mv k3s.yml /home/gitpod/.kube/config`)

This prevent us from accessing core-dev later.

Proposed solution:

• all invocations of kubectl should have an explicit --kubeconfig <kubectxfile> arg.
• there should not be a default kubecontext so that invocations of kubectl without explicit --kubeconfig <kubectxfile> fail. This is for robustness: don't accidentally use the wrong context.