EC2 instance creation fails when specifying an existing network interface

[kylewuolle]

/kind bug

1. Create a cluster where a worker machine has an existing network interface specified
2. awsmachine_controller controller fails to create the worker instance with the following error:
   awsmachine_controller.go:536] "unable to create instance" err=< failed to create AWSMachine instance: failed to run instance: InvalidParameterValue: You cannot specify tags for network interfaces if there are no network interfaces being created by the request.

What did you expect to happen:
I expect the ec2 instance to be created successfully.

Anything else you would like to add:
I have created a PR (#5595) which solves the problem by not adding the tags for network interfaces when providing an existing network interface.

Environment:

• Cluster-api-provider-aws version: v2.8.4
• Kubernetes version: (use kubectl version):
  Client Version: v1.32.2 Kustomize Version: v5.5.0 Server Version: v1.33.1
• OS (e.g. from /etc/os-release): ubuntu 24.04

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