Wrong subjects returned when storing in grafeas

[renzodavid9]

Expected Behavior

The subjects used to calculate grafeas occurrences should be calculated according to the formatter version used. The way v2alpha3 and v1 calculate subjects is different from the way v2alpha4 does it.

Actual Behavior

When using grafeas as a backend, and payload formatter v2alpha4, the subjects from the provenance, used to calculate occurrences, are not right, the subjects are calculated using the same logic all the time, without considering the formatter version:

https://github.com/tektoncd/chains/blob/badc7340a7c749979af6e02e545d870b554830af/pkg/chains/storage/grafeas/grafeas.go#L256

Steps to Reproduce the Problem

1. Configure grafeas backend

2. Using this workflow:

   
   apiVersion: tekton.dev/v1alpha1
   kind: StepAction
   metadata:
   name: stepaction
   spec:
   results:
   - name: art1-ARTIFACT_OUTPUTS
     type: object
     properties:
       uri: {}
       digest: {}
       isBuildArtifact: {}
   - name: art1-other-ARTIFACT_OUTPUTS
     type: object
     properties:
       uri: {}
       digest: {}
       isBuildArtifact: {}
   image: bash:latest
   script: |
   echo -n "{\"uri\":\"gcr.io/foo/img02\", \"digest\":\"sha256:f7b93fed74352c53e7fd82f1e890723f04a963fcd344c65855c57a1972fbe02d\", \"isBuildArtifact\": \"true\"}" > $(step.results.art1-ARTIFACT_OUTPUTS.path)
   echo -n "{\"uri\":\"gcr.io/foo/img03\", \"digest\":\"sha256:fc1f2bab34c4cc65abcdcafd34835f1928663458340bca0e9d61eee39efb4842\", \"isBuildArtifact\": \"true\"}" > $(step.results.art1-other-ARTIFACT_OUTPUTS.path)
   ---
   apiVersion: tekton.dev/v1
   kind: Task
   metadata:
   name: task1
   spec: 
   results:
   - name: art2-ARTIFACT_OUTPUTS
     type: object
     properties:
       uri: {}
       digest: {}
       isBuildArtifact: {}
   
   steps:
   - name: step
     ref:
       name: stepaction
   
   - image: bash:latest
     script: |
       echo -n "{\"uri\":\"gcr.io/foo/img05\", \"digest\":\"sha256:21ff370d4db229a025067f52f87ac9c1bead8cd1068b72febadfa3e45f377101\", \"isBuildArtifact\": \"true\"}" > $(results.art2-ARTIFACT_OUTPUTS.path)
   ---
   apiVersion: tekton.dev/v1
   kind: Pipeline
   metadata:
   name: pipeline
   spec:
   results:
   - name: art4-ARTIFACT_OUTPUTS
     value: $(tasks.task2.results.art3-ARTIFACT_OUTPUTS)
   tasks:
   - name: task1
     taskRef:
       name: task1
   
   - name: task2
     taskSpec:
       results:
         - name: art3-ARTIFACT_OUTPUTS
           type: object
           properties:
             uri: {}
             digest: {}
   
       steps:
         - name: step
           ref:
             name: stepaction
   
         - image: bash:latest
           script: |
             echo -n "{\"uri\":\"gcr.io/foo/img02\", \"digest\":\"sha256:f7b93fed74352c53e7fd82f1e890723f04a963fcd344c65855c57a1972fbe02d\"}" > $(results.art3-ARTIFACT_OUTPUTS.path)
   ---
   apiVersion: tekton.dev/v1
   kind: PipelineRun
   metadata:
   name: pipeline-run
   spec:
   pipelineRef:
   name: pipeline

4. After generating the attestation you will see in the `deployment/tekton-chains-controller` logs that only 3 occurrences  were uploaded, it should be 4.