Disable provenance attestations (#2265)
Docker desktop w/ containerd enabled is creating provenance attestations with each build. When this happens, each image is pushed with 2 additional manifests, one is the provenance manifest containing build facts, and the other an index pointing to the actual image and the provenance manifest. While useful in some contexts, cog is not one of them atm. The registry is ignoring them, but this PR disables creation entirely.
