glsql: Unify paths to retrieve database configuration

While our Postgres-based tests know to extract information about how to connect to the database from gdk env, the code which sets up the Praefect proxy will only use values derived from the environment. This means that users have to manually call the likes of eval $(gdk env) before running "test-with-praefect".

Fix this issue by unifying code paths such that the Praefect proxy setup uses the same code path as the Postgres-based tests.