feat: wg-easy scope CLUSTER_NAME with USER

[nvanthao]

Update Cluster Naming

If there are multiple people working on wg-easy project from platform-examples, the test-cluster will be shared by everyone.

Changes

• Modified default cluster name to use a dynamic format: {username}-cluster (falls back to wg-easy-dev-cluster if USER env var not set)
• Simplified kubeconfig file path to use a static name test-cluster.kubeconfig instead of dynamic cluster name

Why

• Makes cluster names more unique by incorporating username, reducing potential conflicts
• Simplifies kubeconfig file handling by using a consistent filename
• Improves developer experience by making it easier to identify clusters in shared environments

Testing

• Verified cluster creation with new naming convention
• Confirmed kubeconfig file is properly generated and accessible

[banjoh]

It works!

$ task cluster-create
...
Creating new cluster evans-cluster...