Fix BuildKit and BuildAh image digest extraction

[SaschaSchwarze0]

Changes

There was a change recently in BuildKit: its exported metadata file was changed to be formatted. Previously it was a single line JSON. Now, it is a formatted JSON. That required a change to the logic to extract the digest.

I am also changing the e2e test to verify that a successful BuildRun contains a valid image digest. This would have captured this problem earlier.

And that revealed that the digest we stored for BuildAh has always been incorrect. I basically observe the same as here: Digests on local buildah different from registry, unclear how to 'correct' #3866. I am putting in some workaround.

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Set a kind label on this PR
• Release notes block has been filled in, or marked NONE

Release Notes

The BuildKit and BuildAh sample build strategies were fixed to correctly set the `shp-result-image-digest` system result.

[gabemontero]

/approve

I've also pinged a buildah team member about the item @SaschaSchwarze0 noted ... awaiting feedback

[openshift-ci]

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[qu1queee]

Nice. I left a comment on Buildah, I cannot properly judge if the proposed changes on the strategy are the right thing to do, asking to move this into an Issue. We can work on this offline if needed. The rest of the PR can go in as it enhance e2e.

[qu1queee]

/lgtm