This is more of a suggestion but it seems like a lot of people are having problems with sbt's thin client mode (see sbt/sbt#6468). An issue I just found is that currently when SBT runs with --client it ignores all environment variables which makes the env field for WorkflowStep.Sbt not even work (you can see the PR at iheartradio/ficus#155 where I had to set this to false because sbt-coveralls cannot even pick up the env vars)
My suggestion would be to set it completely to false until the problems with a new SBT release.