Explicitly promote installer config as env vars

[acelaya]

Right now, installer config is promoted as env vars as if it was a regular config file. This is not entirely correct, as it does not really return any config at all to merge with the rest of the app config, but instead it reads the config, and sets the values as env vars if they are not set yet.

The reason it's done like this is likely because in the past, the installer config was in fact returning an array to merge with the rest of the config.

Right now it makes no sense to do it like that, and instead it should be an explicit step done right before the config is loaded. Issues like #2201 demonstrate it's easy to forget when are those env vars set because they are not explicitly loaded.