Fix update-context to use KUBECONFIG when the env is set

[govargo]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When user executes minikube update-context with KUBECONFIG env, the command fails.

KUBECONFIG=/tmp/.somewhereelse/config minikube update-context
💣 update config: Kubeconfig does not have a record of the machine cluster
😿 Sorry that minikube crashed. If this was unexpected, we would love to hear from you:
👉 https://github.com/kubernetes/minikube/issues/new/choose

This is because the update-context uses only $HOME/.kube/config(KUBECONFIG env will be ignored).
This PR allows users to use KUBECONFIG env when they execute minikube update-context
If the KUBECONFIG env is not set, $HOME/.kube/config filepath will be used.

Which issue(s) this PR fixes:

Fixes #5944

Does this PR introduce a user-facing change?

Yes.
This PR fixes bug when the user executes KUBECONFIG=xxxx minikube update-context

Before this PR, KUBECONFIG=xxxx minikube update-context command may fail(update-context uses $HOME/.kube/config)

KUBECONFIG=/tmp/.somewhereelse/config minikube update-context
💣 update config: Kubeconfig does not have a record of the machine cluster
😿 Sorry that minikube crashed. If this was unexpected, we would love to hear from you:
👉 https://github.com/kubernetes/minikube/issues/new/choose

After this PR, KUBECONFIG=xxxx minikube update-context command successes

KUBECONFIG=/tmp/.somewhereelse/config minikube update-context
minikube IP has been updated to point at xxx.xxx.xxx.xxx
or
minikube IP was already correctly configured for xxx.xxx.xxx.xxx

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NONE

[k8s-ci-robot]

Hi @govargo. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: govargo
To complete the pull request process, please assign ra489
You can assign the PR to them by writing /assign @ra489 in a comment when ready.

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[minikube-bot]

Can one of the admins verify this patch?

[govargo]

/assign @RA489

[tstromberg]

/ok-to-test

[tstromberg]

Thank you!