Allow the use of variables in the 'deployment' field of a BB Deployment

Bitbucket Pipelines supports user-provided variables: https://bitbucket.org/blog/adding-human-interaction-back-into-automated-builds

However, despite what that page suggests, those variables currently cannot be used in the 'deployment' field. If I try to do this:

custom:
  deploy-to-any-env:
    - variables:
      - name: ENVIRONMENT
    - step: 
        deployment: $ENVIRONMENT
        script: ...
    

Then I get this error:

Therefore, if I have multiple deployment environments and I want to write a custom pipeline that deploys to one of them, I have to actually write many custom pipelines, one per target deployment environment.

Please consider allowing variables in the 'deployment' field.