build: set build ref in response

[crazy-max]

Sets buildx.build.ref in the metadata file.

$ docker buildx build . --metadata-file md.json
...
$ cat md.json
{
  "buildx.build.ref": "default/default/0fjb6ubs52xx3vygf6fgdl611",
  "containerimage.config.digest": "sha256:9651cc2b3c508f697c9c43b67b64c8359c2865c019e680aac1c11f4b875b67e0",
  "containerimage.digest": "sha256:9651cc2b3c508f697c9c43b67b64c8359c2865c019e680aac1c11f4b875b67e0"
}

$ docker buildx bake validate --metadata-file md.json
...
$ cat md.json
{
  "lint": {
    "buildx.build.ref": "default/default/kamngmcgyzebqxwu98b4lfv3n"
  },
  "validate-docs": {
    "buildx.build.ref": "default/default/w1spi3ss9vnatnkznc4fw0btd"
  },
  "validate-vendor": {
    "buildx.build.ref": "default/default/lc2ohlly0xkvop06y81fzyiag"
  }
}