fix arm64 ami build

[Issacwww]

Issue #, if available:

Description of changes:
run into the following issue when testing arm64 ami build

> make 1.28 os_distro=al2023 arch=arm64 
---
The architecture 'x86_64' of the specified instance type does not match the architecture 'arm64' of the specified AMI. Specify an instance type and an AMI that have matching architectures, and try again. You can use 'describe-instance-types' or 'describe-images' to discover the architecture of the instance type or AMI.

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Testing Done
build arm64 ami succeed ami-000353e26e1f56762

See this guide for recommended testing for PRs. Some tests may not apply. Completing tests and providing additional validation steps are not required, but it is recommended and may reduce review time and time to merge.

[cartermckinnon]

What's the reason for this change?

[Issacwww]

instance_type will only been updated if it is not already set when using ?=

thus run into the following error initially

 make 1.28 os_distro=al2023 arch=arm64         

PACKER_ARGS: -var-file templates/al2023/variables-default.json  **-var instance_type='m5.large'** -var arch='arm64'  -var kubernetes_version='1.28.5'

[ndbaker1]

oh but we want to avoid locking in instance_type, any conflicts with this?

ifeq ($(arch), arm64)
	instance_type ?= m6g.large
	AMI_VARIANT := $(AMI_VARIANT)-arm64
else
	instance_type ?= m5.large
endif

[Issacwww]

ah, it would be better, although it has been defaulted to m5.large at line 19 already 😄
will update it in next rev shortly