`gke-gcloud-auth-plugin`: Support overriding `gke_gcloud_auth_plugin_cache` location

[erebe]

Following #554

Issue:

Currently, gke-gcloud-auth-plugin create/use a cache at a fixed location in $HOME/.kube/gke_gcloud_auth_plugin_cache
This cause issue when several accesses using different account is needed, because gke-gcloud-auth-plugin always endup using this cache if present

It would really be nice to have an environment variable to change the location of this cache at least.
As example, when running inside a CI, the fixed path break the isolation per run.
The only workaround is to change HOME variable before executing the command, but this feel unsafe to do with potentially un-intended side effects.

Proposed solution:

Introduce a parameter or an environment variable to change the location of this cache

[k8s-ci-robot]

This issue is currently awaiting triage.

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