I don’t think the liquibase plugin support AwsCredentials as which can be done when defining repositories, so you have some options.
Environmental variables:
username = System.getenv('AWS_KEY') ?: ''
password = System.getenv('AWS_SECRET') ?: ''
or investigate using one of the many credentials plugins - https://plugins.gradle.org/search?term=credentials. I have used @etiennestuder 's version many times - https://plugins.gradle.org/plugin/nu.studer.credentials