Update location of blubber's buildkit frontend image
Latest location of blubber image is repos/releng/blubber/buildkit,
where the last buildkit portion of the path was recently added.
Also updated the version to v0.12.2
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Latest location of blubber image is repos/releng/blubber/buildkit,
where the last buildkit portion of the path was recently added.
Also updated the version to v0.12.2