getdb: Allow overriding the environment that is being pulled from

[dalin-]

I need to pull the DB from a multidev. I know that's going to be faster than trying to reproduce some complicated test content that it has. It'd be great if I could just do something like

fire getdb pr-123

or

fire getdb --env=pr123

(I'm not sure which style better fits how the other fire commands work)

I'm not sure if it should save to a differently named SQL file, or the standard. I'm guessing the former.