sg: Fix migration command connecting to default database
Created by: mrnugget
This fixes the regression introduced in 11239b098b362cc4d00d53da7308daeb3c987933.
Before 11239b0 we didn't need a prefix for the PG*
env vars to
connect to the default database, meaning that all the PGUSER
env vars
etc. would be used unchanged.
11239b0 removed the prefix defaulting to "" which broke that behaviour.
This reintroduces it.