Install only mode

It would be nice to have a way to use this action without the need of creating a cluster, only for install k3d purposes. Imagine a user who wants to start `n` of k8s clusters and the `n` is not known in advance but it's dynamically evaluated during the build or whatever.

The officially suggested way of `curl -s https://raw.githubusercontent.com/k3d-io/k3d/main/install.sh | bash
` isn't super secure.

Something like `skipClusterCreation: true`,`installOnly: true`, `dontCreate: true` or `create: false`.