This is quite incomplete - I wanted to get first impressions on the approach. It's basically adding an ImmutableMap<String, String> environment to all the steps that use getCommandPrefix. To some extent I wonder if passing Tool to the steps instead would be better - although I suppose that's a bit harder to test with.
Separately, if you could, let me know whether you prefer a commit per directory or one giant squash. I'm hoping one-per-directory makes it easier to review, but it's definitely a PITA to bisect.
Either way - let me know if this looks good. I'll finish the other ~70 uses once I get the ok :)