It's worth pointing out that if the salt is somehow exposed to an attacker, or is guessable, much of the benefit goes away. It's likely that a timing attack could be used to discover the salt if it is fixed per machine or process over a long period of time.
If a salt is generally fixed per machine, but varies from machine-to-machine, I think we'll see an influx of frustrated devs who have something that works perfectly on their machine but not for others. It doesn't matter that they're doing it wrong, we'll still have to deal with them as a community. This seems like an argument in favor of randomizing it at runtime by default, so it fails early for them.
Allowing an environment and command line override makes sense, as it allows users to rotate the salt as frequently as their needs dictate.