sg migration doesn't use sg.config.yaml to get PG* env vars

Created by: mrnugget

The sg migration command don't use the PG (or other) env vars set in sg.config.yaml.

Why?

sg migration commands use RawDSNsBySchema to construct DSNs:

https://github.com/sourcegraph/sourcegraph/blob/9c700e296acc9cff5cc51c76b1391c46ecf16aca/dev/sg/sg_migration.go#L110-L113

And RawDSNsBySchema uses os.Getenv to get config variables:

https://github.com/sourcegraph/sourcegraph/blob/9c700e296acc9cff5cc51c76b1391c46ecf16aca/internal/database/postgresdsn/schema.go#L36-L38

How to fix

What we would need to do instead is to parseConf and then pass globalConf.GetEnv when constructing DSNs, like we do here, for example:

https://github.com/sourcegraph/sourcegraph/blob/9c700e296acc9cff5cc51c76b1391c46ecf16aca/dev/sg/sg_db.go#L71-L82

We probably also want a fallback to os.Getenv in case we can't parse the config file.