Always overwrite env vars

Since unicorn zero downtime doesn't work if we don't overwrite env vars - the new master inherits the old one's env and DotEnv dutifully doesn't overwrite the old value which leads to undeployable configuration.

I don't see a reason to not overwrite env vars, even in development. Why wouldn't you use DotEnv and have everything in the same place, why litter your env with vars from all kinds of places. DotEnv solves that mess i.m.o.