chore: use docker buildx

[alexghr]

Looks like the recent AMI change has changed the way docker pushes images. They are manifests now and the docker manifest create can't flatten a manifest in a manifest:

# yesterday's image
$ docker manifest inspect aztecprotocol/aztec:4.0.0-nightly.20260119-amd64 | jq .mediaType
"application/vnd.docker.distribution.manifest.v2+json"

# today's image
$ docker manifest inspect aztecprotocol/aztec:4.0.0-nightly.20260120-amd64 | jq .mediaType
"application/vnd.oci.image.index.v1+json"

CI logs from the nightly build:

04:16:46 Waiting for amd64 image to be pushed...
04:16:56 Waiting for amd64 image to be pushed...
04:17:08 docker.io/aztecprotocol/aztec:4.0.0-nightly.20260120-amd64 is a manifest list
04:17:08 + cleanup
04:17:08 + set +e
04:17:08 + '[' -n '' ']'
04:17:08 + '[' -n '' ']'
04:17:08 + '[' -n '' ']'
04:17:08 + local code=1