server: unable to load custom python environment with python backend

I’m trying to use a custom environment for a pytorch model served with the python backend this is the config file

name: "model1"
backend: "python"

input [
  {
    name: "INPUT0"
    data_type: TYPE_FP32
    dims: [ 3 ]
  }
]
output [
  {
    name: "OUTPUT0"
    data_type: TYPE_FP32
    dims: [ 2 ]
  }


instance_group [{ kind: KIND_CPU }]

parameters: {
  key: "EXECUTION_ENV_PATH",
  value: {string_value: "$$TRITON_MODEL_DIRECTORY/model1/python-3-8.tar.gz"}
}

The file structure is like this

|-- model1
|   |-- 1
|   |    -- model_ckpt.pb
|   |   `-- model.py
|   |-- config.pbtxt
|   |-- python-3-8.tar.gz
|   `-- triton_python_backend_stub

I’m getting the error UNAVAILABLE: Internal: Failed to get the canonical path for $$TRITON_MODEL_DIRECTORY/model1/python-3-8.tar.gz.

Please help !