Make AWS AMI builder fetch package from artifact instead of S3 bucket

What does this MR do?

Make AWS AMI builder fetch package from artifact instead of S3 bucket. Depends on !6973 (merged).

Because testing this locally (with CI_JOB_TOKEN) is difficult, to be on the safer side I have added an env variable switch that will help us use the old way of building it in case we see some breakage during the release. This involves maintaining two copies of the files, but given we can remove them once we see the release, I think it is an acceptable tradeoff.

Related issues

Closes: https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/7840

Checklist

See Definition of done.

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Required

• MR title and description are up to date, accurate, and descriptive.
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For GitLab team members

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• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
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Expected (please provide an explanation if not completing)

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