fix: Changed non portable usage of base64 --decode flag to -d

[joeyqliu]

What does this PR do?

• Changed coredns local exec calls to base64 with flag --decode to -d which is supported universally on darwin & other linux distributions.

Motivation

• Resolves Non-portable usage of base64 decode via --decode flag #1115

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• [n/a] Yes, I have added a new example under examples to support my PR
• [n/a] Yes, I have created another PR for add-ons under add-ons repo (if applicable)
• [n/a] Yes, I have updated the docs for this feature
• Yes, I ran pre-commit run -a with this PR

Note: Not all the PRs require a new example and/or doc page. In general:

• Use an existing example when possible to demonstrate a new addons usage
• A new docs page under docs/add-ons/* is required for new a new addon

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

This TF code is being run on alpine based system, not MacOS. Demonstrates changes fixed the initial issue.

module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0]: Destruction complete after 0s
module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0]: Creating...
module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0]: Provisioning with 'local-exec'...
module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].null_resource.remove_default_coredns_deployment[0]: Creation complete after 1s [id=5577006791947779410]
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0]: Creating...
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0]: Provisioning with 'local-exec'...
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Creating...
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0] (local-exec): (output suppressed due to sensitive value in config)
module.eks_blueprints.module.aws_coredns[0].null_resource.modify_kube_dns[0]: Creation complete after 0s [id=8674665223082153551]
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Still creating... [10s elapsed]
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Still creating... [20s elapsed]
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Still creating... [30s elapsed]
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Still creating... [40s elapsed]
module.eks_blueprints.module.aws_coredns[0].module.helm_addon[0].helm_release.addon[0]: Creation complete after 43s [id=coredns]

Apply complete! Resources: 3 added, 0 changed, 1 destroyed.```

[askulkarni2]

Thanks for the PR. LGTM!