fix(backend): fix cache server output (kubeflow#1018)

Author: Tomcli

backend/src/cache/server/mutation.go

@@ -297,7 +297,7 @@ func mutateContainer(results map[string][]*tektonv1beta1.TaskRunResult, original
 	args := []string{}
 	args = append(args, "printf 'This step output is taken from cache.\n\n'")
 	for _, result := range outputs {
-		arg := fmt.Sprintf("printf '%s: %s\n'; printf '%s' > /tekton/results/%s", result.Name, result.Value, result.Value, result.Name)
+		arg := fmt.Sprintf("printf '%s: %s\n'; printf '%s' > /tekton/results/%s", result.Name, result.Value.StringVal, result.Value.StringVal, result.Name)
 		args = append(args, arg)
 	}
 
@@ -335,7 +335,6 @@ func unmarshalResult(taskResult string) (map[string][]*tektonv1beta1.TaskRunResu
 	if err != nil {
 		return nil, err
 	}
-
 	return results, nil
 }