Handle GitHub action cancellation gracefully.

#### Community Note

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#### Description

When packer is invoked in GitHub action, if the job is cancelled, it may not shutdown resources associated with the build. This will cause the compute instance to remain on forever and incur a significant bill.

#### Use Case(s)

Run Packer in any CI pipeline

#### Potential configuration

Provide a github action handler for packer build step, and ensure it can always clean up resource with a post-run session.

#### Potential References
https://github.com/orgs/community/discussions/26311

No source files, tests, or entry points are named. Start by tracing Packer's build shutdown lifecycle and GitHub Actions cancellation behavior, then determine how a post-run session could reliably clean up resources. Done means cancelled builds do not leave associated compute instances running.

Written by the indexing model from the issue text.