I think we should limit the number of environment variables we use in production - only things that we actually change should get a unix env var. Things we might want to change at runtime can be OTP application env vars (specified in .app.src) - that's fine. If we change something often enough, it gets promoted to a unix env var (or if there is no good default value - like the location of a particular redis server or something).

It's really only variables that will be different per-cluster that should get unix env vars - I don't see these timing choices as being that.