Unable to use user variables in pausing_before

[manikantakrishna]

{
"variables": {
    "FirstProvisionTime": "",
     "RestOfProvisionTime": ""
  },
  "provisioners": [
    {
      "type": "windows-restart",
      "pause_before": "{{ user `FirstProvisionTime` }}",
      "restart_timeout": "{{ user `FirstProvisionTime` }}"
    },
   {
      "type": "powershell",
      "pause_before": "{{ user `RestOfProvisionTime` }}",
      "script": "scripts/script.ps1",
      "elevated_user": "vagrant",
      "elevated_password": "vagrant"
    },
}

Error
ui error: Failed to parse template: 1 error(s) occurred:

• provisioner 2: 1 error(s) decoding:

• error decoding 'pause_before': time: invalid duration {{ user FirstProvisionTime }}

• provisioner 1: 1 error(s) decoding:

• error decoding 'pause_before': time: invalid duration {{ user RestOfProvisionTime }}

• provisioner 2: 1 error(s) decoding:

I'm getting error for pause_before value.
restart_timeout value is passing successfully, Why I'm not able to pass user variables in pause_before.

[SwampDragons]

Duplicate of #5115