Can't overwrite a KUBECONFIG that's in use

[dhaiducek]

This line will cause issues if a user is still using the file to login to the ClusterPool cluster and reuses the same claim:
https://github.com/open-cluster-management/lifeguard/blob/0ce949b7e2d9997a1d5e0511e3b8ba4af946c165/clusterclaims/apply.sh#L389

[dhaiducek]

Should be fairly straightforward (?) to check up-front whether the KUBECONFIG in use is a lifeguard/clusterclaims/*/kubeconfig file

[dhaiducek]

I've addressed it in startrhacm this way:

if [[ "${KUBECONFIG}" == */lifeguard/clusterclaims/*/kubeconfig ]]; then
  printlog error "KUBECONFIG is set to an existing claim's configuration file. Please unset before continuing: unset KUBECONFIG"
  exit 1

https://github.com/dhaiducek/startrhacm/blob/e271da0a72ce8ac0ea4b4eb86e2936e68e77219d/startrhacm.sh#L70-L72