Kserve defaulting causing duplicates of environment variable

/kind bug

What steps did you take and what happened: Create example xgboost isvc and enable gRPC

apiVersion: "serving.kserve.io/v1beta1"
kind: "InferenceService"
metadata:
  name: "xgboost-iris"
spec:
  predictor:
    xgboost:
      protocolVersion: "v2"
      storageUri: "gs://kfserving-examples/models/xgboost/iris"
      ports:
      - containerPort: 9000
        name: h2c
        protocol: TCP

The pod spec has duplicated environment variable

    Environment:
      MLSERVER_MODEL_NAME:            xgboost-iris
      MLSERVER_MODEL_URI:             /mnt/models
      MLSERVER_MODEL_NAME:            xgboost-iris
      MLSERVER_MODEL_URI:             /mnt/models

Additionally, attempt to override the defaults leads to duplicated environment variable with different values

apiVersion: "serving.kserve.io/v1beta1"
kind: "InferenceService"
metadata:
  name: "xgboost-iris"
spec:
  predictor:
    xgboost:
      protocolVersion: "v2"
      storageUri: "gs://kfserving-examples/models/xgboost/iris"
      ports:
      - containerPort: 9000
        name: h2c
        protocol: TCP
      env:
      - name: MLSERVER_MODEL_NAME
        value: my-model

The pod spec:

    Environment:
      MLSERVER_MODEL_NAME:            my-model
      MLSERVER_MODEL_NAME:            xgboost-iris
      MLSERVER_MODEL_URI:                /mnt/models

What did you expect to happen:

• Defaulting should not duplicate environment variable and should prioritise user's defined environment variable

Anything else you would like to add: I believe it's because the defaulter always append .Env without checking the presence of existing environment variable. (https://github.com/kserve/kserve/blob/a6ed8e4b006e27433de2336e0e8b7cec11137dc1/pkg/apis/serving/v1beta1/inference_service_defaults.go#L264)

Environment:

• Kserve: 0.8.0