Skip caching if dir doesn't exist

[spowelljr]

Fixes #102

Check if the directory exists before trying to cache it, this will prevent error messages that can confuse the user. This is usually caused because we try caching the kicbase and ISO, but only one will exist, so the non-existing one throws a message when it fails to cache.

Before:

* Done! kubectl is now configured to use "minikube" cluster and "default" namespace by default
/home/runner/bin/minikube version --short
v1.29.0
isoError: Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.

After:

* Done! kubectl is now configured to use "minikube" cluster and "default" namespace by default
/home/runner/bin/minikube version --short
v1.29.0