Instantiate a network to ensure the resource group is deleted

AzureNetwork generally handles the lifecycle of the Azure resource group. With recent #6442 no longer instantiating an AzureVirtualMachine during AksCluster initialization, a network was no longer being instantiated. Apparently this was critical code, as `az group delete` was no longer being called. Added it back in & deletion is working again.

Thanks to @vofish for pointing out the error.

PiperOrigin-RevId: 872013241

Author: hubatish

perfkitbenchmarker/providers/azure/azure_kubernetes_service.py

@@ -126,6 +126,8 @@ def __init__(self, spec):
     # to make it work with ACR
     self.cluster_version = FLAGS.container_cluster_version
     self._deleted = False
+    # Instantiation required to delete the resource group.
+    self.network = azure_network.AzureNetwork.GetNetwork(spec.vm_spec)
 
   def InitializeNodePoolForCloud(
       self,