Using an environment variable inside "add_ssh_keys -> fingerprints" commands

Hi there. I tried use add_ssh_keys command in the way describing in topic, but job log says “There are no configured ssh keys to install”.
Documentation says it’s required step, even with using “manual ssh key instaling”. And it’s true, without fingerprint, job has failed everytime. Could anyone help with this problem please?

Part of my config:

    executor: node
      - attach_workspace:
          at: ./
      - add_ssh_keys:
            - $GH_FNPR
      - run:
          name: Add to known hosts
          command: ssh-keyscan >> ~/.ssh/known_hosts

Oh, it seems like @hermant2 faced with this problem too( in this topic)