Add environment variables support

[ZAlexanderV]

Is it possible to fix environment variable support for Doppler configuration?
Currently, I have to get the variables at the beginning of the book and then insert them into the search plugin, which is not at all convenient.
If I run it hoping that the environment variable DOPPLER_TOKEN will be picked up, I'll get an error:

 {"msg": "An unhandled exception occurred while running the lookup plugin 'dcostakos.doppler.doppler_secrets'. Error was a <class 'TypeError'>, original message: '_Environ' object is not callable. '_Environ' object is not callable"}

Here is my step

    - name: Test lookup plugin
      ansible.builtin.debug:
        msg: "{{ lookup('dcostakos.doppler.doppler_secrets', project='doppler', config='dev', name='APIKEY') }}"

I exported DOPPLER_TOKEN before start playbook.