Skip EBS AMI preparation when no image is created

[FocalChord]

Description

This changes the Amazon EBS builder so skip_create_ami = true also reaches the source instance AMI preparation steps.

Today those steps can still stop the source instance and modify source instance attributes before the create image step observes skip_create_ami. For no image builds, I think those steps are AMI preparation work and can be skipped.

The patch passes skip_create_ami into:

1. StepStopEBSBackedInstance
2. StepModifyEBSBackedInstance

This PR intentionally does not change the final source instance termination wait. That behavior is a separate cleanup concern and can be discussed independently.

Resolved Issues

Closes #680

Evidence

I tested this with no image Amazon EBS runs that collect outputs before cleanup.

1. Full no image EBS run on c5.metal with Amazon plugin 1.8.1
   The source instance stop wait took about 19m16s.

2. Local patch that skips source stop and source attribute preparation
   Packer logged that source instance stop was skipped when skip_create_ami = true.

3. The remaining cleanup wait moved to final termination, which is why this PR avoids addressing that separate behavior.

Validation

1. go test ./builder/common ./common ./builder/ebs
2. Git whitespace check on the amended commit

Rollback Plan

Revert this change.

Changes to Security Controls

No. This does not change access controls, encryption, or logging.

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[Copilot]

Pull request overview

This PR updates the Amazon EBS builder workflow so that when skip_create_ami is enabled, AMI-preparation steps (stopping the source instance and modifying source instance attributes) are skipped instead of running before AMI creation is bypassed.

Changes:

• Plumbs AMISkipCreateImage into the EBS builder’s source-instance stop and attribute-modification steps.
• Adds early-exit behavior in the stop/modify steps to skip work when skip_create_ami is set.
• Adds unit tests covering the new early-exit behavior for both SDK v2 (common/) and SDK v1 (builder/common/) step implementations.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
common/step_stop_ebs_instance.go	Skips stopping the source instance when AMISkipCreateImage is true.
common/step_stop_ebs_instance_test.go	Adds a test ensuring stop is skipped when skip_create_ami is set.
common/step_modify_ebs_instance.go	Skips source instance attribute modification when AMISkipCreateImage is true.
common/step_modify_ebs_instance_test.go	Adds a test ensuring modification is skipped when skip_create_ami is set.
builder/ebs/builder.go	Passes AMISkipCreateImage into the stop/modify steps in the EBS builder step chain.
builder/common/step_stop_ebs_instance.go	Mirrors skip behavior for the SDK v1 step implementation.
builder/common/step_stop_ebs_instance_test.go	Adds a test ensuring stop is skipped (SDK v1 implementation).
builder/common/step_modify_ebs_instance.go	Mirrors skip behavior for the SDK v1 modification step.
builder/common/step_modify_ebs_instance_test.go	Adds a test ensuring modification is skipped (SDK v1 implementation).

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