Install only mode

[jkremser]

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.