Allow customizing AMI of eksctl Linux/Windows nodes

[ConnorJC3]

What type of PR is this?

/kind feature

What is this PR about? / Why do we need it?

This PR adds the ability to customize the AMI of linux and windows nodes in make cluster/create (for eksctl only atm).

How was this change tested?

$ export CLUSTER_TYPE=eksctl  
$ export WINDOWS=true  
$ export LINUX_AMI=ami-AMI_ID_HERE
$ export WINDOWS_AMI=ami-AMI_ID_HERE
$ make cluster/create

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

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[github-actions]

Code Coverage Diff

This PR does not change the code coverage

[ElijahQuinones]

Wouldn't this make it so the a1compat test will no longer be able to be run as it uses al2 as a1 instance types are incompatible with al2023 https://docs.aws.amazon.com/linux/al2023/ug/ec2.html

[ConnorJC3]

Yes, you're right, we'll need to override specifically for that test - I'll update prow-e2e.sh in the next revision

[k8s-ci-robot]

PR needs rebase.

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