Prevent ansible-init running during packer build

[wtripp180901]

When running a Packer build which is based on a "fat image", ansible-init runs during boot of the Packer build VM. This is undesirable as:

• In the general case it may take actions which are not appropriate during a build
• When ansible-init completes it writes its sentinel file into the resulting image, which means booting a cluster instance using the resulting image never runs ansible-init

This PR disables ansible-init during packer builds via a metadata flag.

[sjpb]

@wtripp180901 can you update from main please?

[bertiethorpe]

Latest image tested by:

• New fat image built
• 'extra' build on top of it
• mount that image to make sure ansible-init hasn't run during the second fatimage build
• then launch the image in a VM to make sure the sentinel file is written

New image passing these tests.
https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/11451000682

[sjpb]

@bertiethorpe there is no updated reference to ansible-init including azimuth-cloud/ansible-collection-image-utils#1

[sjpb]

LGTM