Implement environment variables to redirect FS access

[mmastrac]

Discussed offline.

We should implement two environment variables that will redirect the FS access for two parts of the gel-rust driver (and eventually, the other drivers as well).

This will allow us to better isolate driver and CLI tests.

One envvar will redirect the ~/.local/share/edgedb/ directory and the other will redirect ~/.config/edgedb/.

Variable names TBD.