EBS builds with skip_create_ami still run AMI preparation steps

### Overview

I think `skip_create_ami` no image runs can avoid the source instance AMI preparation steps.

The order I observed was:

1. Source instance stop starts.
2. Packer waits for the instance to stop.
3. The create image step later handles `skip_create_ami`.

For workflows that use Packer as remote execution and collect outputs before cleanup, stopping the source instance for AMI preparation adds time without producing an image.

### Reproduction shape

1. Run an Amazon EBS build with `skip_create_ami = true`.
2. Copy outputs during the build.
3. Run with `PACKER_LOG=1`.

Observed order:

```text
==> example.amazon-ebs.test: Stopping the source instance...
==> example.amazon-ebs.test: Waiting for the instance to stop...
==> example.amazon-ebs.test: Skipping AMI creation...
```

### Measurements

The source instance stop wait in a `c5.metal` run took about `19m16s`.

A local patch that passes `skip_create_ami` into the stop and modify steps skipped that stop wait. A separate cleanup wait remained, which I think should be treated as a separate concern.

### Expected behavior

My theory is that `skip_create_ami = true` can skip AMI preparation steps because no image will be created.

### Versions

Packer `1.15.1` for local plugin validation.
Amazon plugin `1.8.1` for the released plugin comparison.
Local plugin build from current `main` for the proposed patch.