Make cache-dir configurable wih env variable

[strigazi]

What would you like to be added:
Make kubectl use an optional environment variable to configure the cache-dir.

https://kubernetes.io/docs/reference/kubectl/kubectl/
--cache-dir string     Default: "$HOME/.kube/cache"
Default cache directory

Why is this needed:
At the moment, users have to pass the --cache-dir option on every kubectl execution, with an environment variable (similar to KUBECONFIG), they will have to set it once.

[ardaguclu]

cache-dir is used by discovery and kubectl commands using discovery are api-resource, create and a few others(autoscale, version).There is no need to pass cache-dir in every execution, if you need to customize cache-dir. You can just pass cache-dir for the commands using discovery.

I understand the inconvenience in here(From the user perspective, how user knows which command uses discovery and which one not). But there is an ongoing work on discovery caching. Even though it is small, there is a probability that cache-dir will be removed. At that point, I think we might not want to add new environment variable.

[traylenator]

I'd take a step back as to what we were trying to achieve.

The default cache is $HOME/.kube/cache

We want to be able to make the default cache $XDG_RUNTIME_DIR/kubecache
since it is a much quicker filesystem.

It's probably a better default generally though possibly a bit linuxy "sic" for sure.

[eddiezane]

/assign @ardaguclu
/triage accepted
/priority backlog

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[ardaguclu]

/remove-lifecycle stale