Fix hard-coded user/database used in instance upgrade

[fantix]

• EdgeDB CLI Version: 3.2.0-dev.1009+37ae47d

Steps to Reproduce:

1. Create a custom database, drop the default edgedb database
2. Update the credentials JSON file with the custom database name
3. Run instance upgrade that includes a dump/restore

Expected: success Actual: failure (but edgedb dump ... works fine)

Hard-coded at:

https://github.com/edgedb/edgedb-cli/blob/496b7a7cbdf7fd20329f2d2f32c619dcc4761463/src/portable/local.rs#L359-L360

Maybe we don't need the admin connection for upgrade with dump/restore?

[tailhook]

We need for restore at least. Not sure about dump.