opens is a map, so opens[c.Driver] on a driver this build does not carry
returns a nil function, and gorm.Open calls it. The operator saw a nil
dereference inside gorm with nothing naming the driver.
sqlite3 is the case that bites: it needs cgo and is only compiled in
under the sqlite3 build tag, so the same config file works on one binary
and dies on another. Resolve the driver first and say which ones this
build supports.
