Multiple configs in KUBECONFIG causes file to be saved in incorrect location

[derimagia]

What happened:
KUBECONFIG can be multivalued, see https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/#the-kubeconfig-environment-variable and tsh treats it as one location.

It saves the kubectl config in the literal path E.G. "$HOME/.kube/config:$HOME/test"

What you expected to happen:
Most likely want to emulate what kubectl does and save it in the first file in the list (when change context it sets it in this one).

How to reproduce it (as minimally and precisely as possible):

KUBECONFIG="$HOME/.kube/config:$HOME/test" tsh login --proxy example.com

Environment:

• Teleport version (use teleport version):
  Teleport v3.0.0

• Tsh version (use tsh version):
  Teleport v3.0.0

• OS (e.g. from /etc/os-release):
  macOS 10.14 (18A391)

Browser environment

• Browser Version (for UI-related issues):
• Install tools:
• Others:

Relevant Debug Logs If Applicable

• tsh --debug
• teleport --debug