fix empty provenance metadata for multi-platform builds - #3536

$ docker buildx b --platform linux/amd64,linux/arm64 --metadata-file metadata.json -<<<'from alpine'
#0 building with "default" instance using docker driver
...
#8 resolving provenance for metadata file
#8 DONE 0.0s

$ cat metadata.json 
{
  "buildx.build.ref": "default/default/lx4yc0y6363w40bnrtukr12v1",
  "containerimage.descriptor": {
    "mediaType": "application/vnd.oci.image.index.v1+json",
    "digest": "sha256:bcbe77faeb9f47ba3edb8a60257921eb6be41522b26aeb57b2361eeb4d7d3b14",
    "size": 1607
  },
  "containerimage.digest": "sha256:bcbe77faeb9f47ba3edb8a60257921eb6be41522b26aeb57b2361eeb4d7d3b14",
  "image.name": "moby-dangling@sha256:bcbe77faeb9f47ba3edb8a60257921eb6be41522b26aeb57b2361eeb4d7d3b14"
}

$ docker buildx b --platform linux/amd64,linux/arm64 --metadata-file metadata.json -<<<'from alpine'
#0 building with "default" instance using docker driver
...
#8 resolving provenance for metadata file
#8 DONE 0.0s

$ cat metadata.json 
{
  "buildx.build.provenance/linux/amd64": {
    "builder": {
      "id": ""
    },
    "buildType": "https://mobyproject.org/buildkit@v1",
    "materials": [
      {
        "uri": "pkg:docker/alpine@latest?platform=linux%2Famd64",
        "digest": {
          "sha256": "4bcff63911fcb4448bd4fdacec207030997caf25e9bea4045fa6c8c44de311d1"
        }
      }
    ],
    "invocation": {
      "configSource": {
        "entryPoint": "Dockerfile"
      },
      "parameters": {
        "frontend": "dockerfile.v0",
        "args": {
          "dockerfilekey": "dockerfile"
        },
        "locals": [
          {
            "name": "context"
          },
          {
            "name": "dockerfile"
          }
        ]
      },
      "environment": {
        "platform": "linux/amd64"
      }
    }
  },
  "buildx.build.provenance/linux/arm64": {
    "builder": {
      "id": ""
    },
    "buildType": "https://mobyproject.org/buildkit@v1",
    "materials": [
      {
        "uri": "pkg:docker/alpine@latest?platform=linux%2Farm64",
        "digest": {
          "sha256": "4bcff63911fcb4448bd4fdacec207030997caf25e9bea4045fa6c8c44de311d1"
        }
      }
    ],
    "invocation": {
      "configSource": {
        "entryPoint": "Dockerfile"
      },
      "parameters": {
        "frontend": "dockerfile.v0",
        "args": {
          "dockerfilekey": "dockerfile"
        },
        "locals": [
          {
            "name": "context"
          },
          {
            "name": "dockerfile"
          }
        ]
      },
      "environment": {
        "platform": "linux/amd64"
      }
    }
  },
  "buildx.build.ref": "default/default/m5yq6a7jqtueumuy6el6ntmkh",
  "containerimage.descriptor": {
    "mediaType": "application/vnd.oci.image.index.v1+json",
    "digest": "sha256:3a23d5c73657d7ced2f9c9503006007050a333d43da3a9e16930a262c98590b0",
    "size": 1607
  },
  "containerimage.digest": "sha256:3a23d5c73657d7ced2f9c9503006007050a333d43da3a9e16930a262c98590b0",
  "image.name": "moby-dangling@sha256:3a23d5c73657d7ced2f9c9503006007050a333d43da3a9e16930a262c98590b0"
}