Add possibility to set ENV Variable for assume callback

[kabakakao]

Hello,
we are using the "aws_assume_role" and "aws_mfa" callback.
Now it would be nice, if it would be possible, to set the Role to be assumed within the sfn-parameters file.
I updated our parameter files from .json to .rb. But I didn't get it working to use the ENV Variable.
My example:

require 'attribute_struct'

ENV['AWS_STS_ROLE_ARN'] = 'MyRoleARN'

Configuration.new do
  credentials do
    aws_sts_role_arn 'MyRoleARN'
  end
end

AttributeStruct.new do
  parameters do
  end
  apply_stacks [
    'TEST-STACK'
  ]
  template 'out/my_template.json'
end

Is there a way to get it working?